White noise can also be used to help mitigate the symptoms of illnesses such as tinnitus. To understand why - and how white noise may be able to help you specifically - it would be helpful to first understand precisely what white noise is.
Man staring at static TV with white noise

White Noise: An Overview

To understand what white noise is, we first need to take a closer look at soundwaves, which contain two core elements: 
  • Frequency. How fast the waveform vibrates per second, measured in Hertz (Hz). Human beings can normally hear frequencies ranging from 20 to 20,000 Hz.
  • Amplitude. How loud a sound is, measured in Decibels (Db). Sounds above 70 Db are typically considered harmful to humans, and prolonged exposure to sounds above 85 Db can result in permanent hearing damage. 
diagram explaining the frequency and amplitude of white noise sound waves
What makes white noise unique is that it spans all audible frequencies. In that regard, it's similar to white light, which derives its name in much the same fashion, being an amalgamation of the entire visible spectrum of light. Unlike white light, however, white noise is capable of drowning out its constituent frequencies.

It often has a monotonous sound, similar to the drone of a bedroom fan, the hum of an air conditioner, or the soft static of a radio. If you live in the city, you know how irritating the sounds of typical nightlife can be, especially when you're trying to fall asleep. White noise can help drown out your surroundings and can be especially helpful in putting newborns to sleep. 
It's important to note that different people experience white noise differently. While some people may find it improves their focus and attentiveness, others may find it frustrating and distracting. There may also be personal preferences at play here.
Some people may find the sound of an air conditioner comforting, while others may consider it irritating. Some people may prefer the sound of rain, while others might enjoy nature noises. Others might demand total silence. 

The Spectrum of Sound

As you might expect, white noise is itself part of an entire spectrum of sound, including:
Pink noise
Similar to white noise, but with less sound in higher-frequencies. This gives it a softer sound than typical white noise, and may actually make it even better-suited than white noise for aiding in sleep hygiene. Pink noise occurs frequently in nature, and examples include the sound of rain and the rustling of leaves in the wind.
Brown noise
At the deeper end of the spectrum, we have brown noise. It's characterized by a deep rumbling, as one might hear in a waterfall, the sound of waves, or thunder.
Blue noise
Opposite brown noise on the spectrum, blue noise tends to be quite high-pitched, while having the same rhythm as regular white noise. It's frequently utilized in audio engineering and is characterized by a sort of high-pitched hissing sound. 
Violet noise
Violet noise is unique in that it increases in volume at higher frequencies. It's often used in the treatment of tinnitus, as the frequencies at which it occurs naturally block out the ringing heard by many sufferers. 
Grey noise
A unique type of white noise specifically engineered to assess hearing difficulties. The thing that makes grey noise unique is that it has power at both the top and bottom of the frequency spectrum, with less in the middle, making it sort of an amalgamation of blue noise and brown noise. 
Black noise
Complete silence.

How Does White Noise Work, Exactly?

There are multiple explanations for why white noise works. One theory is that in loud environments, the ear is much less sensitive to sound. For example, a dripping faucet might not be on your radar during the day, but when you're trying to sleep at night and everything is quieter, it might sound as loud as a jet engine.

White noise raises the sound threshold, making these small, sleep-disrupting irritations no longer audible. 
Another explanation involves variability. Because the brain is always on the lookout for acoustic stimuli, unexpected noises may disrupt one's sleep. White noise manifests consistently rather than a series of irregular and potentially-disruptive noises.

It's a little bit like the difference between ordinary lights and strobe lights - the white noise gives your brain something constant to focus on without overwhelming it.
Finally, there's also some evidence that white noise can have a calming effect, easing symptoms of anxiety and helping sufferers calm down their overactive brains. 

Using White Noise to Relax

Having a perfectly-silent bedroom is impossible. There will always be noises, ranging from neighbor's dogs to construction equipment to typical night-time traffic. Because your hearing doesn't shut down entirely while you're asleep, it can be effective to give your brain something to focus on to keep yourself calm and rested. 

Purchasing a white noise machine at a store or online might be a good solution to your problem. These devices produce a range of soothing sounds designed to help improve your sleep quality. Besides improved sleep, you might also find that they reduce stress, increase your focus, soothe headaches and migraines, and even mask tinnitus. White noise has also, on occasion, been used in hypnosis.
White noise generators are also very popular with parents of small children. The monotonous sounds are said to remind an infant of their mother's womb, which has a calming effect. As early as the 6th month of pregnancy, babies are said to be aware of various subtle sounds including muffled tones or their mother's heartbeat.  

Note that if you're using a white noise machine in such a fashion, you should be extremely mindful of setting the volume at an appropriate level. Set them a good distance away from the child, and keep them considerably below 80 Db. White noise should be subtle, not loud. 
If you'd rather not shell out for a white noise machine, there are also plenty of white noise apps for smartphones and tablets. One example is White Noise Lite on Android and iOS, which can produce a number of customizable white and pink noise audio-scapes. Most such apps include a range of different noise colors and styles, including fans, air conditioners, rushing water, ocean waves, and more.

Keeping the Noise Down

White noise has many applications, ranging from therapeutic to medical. While it may not work for everyone, using it in your own life can help improve sleep quality, reduce anxiety, and even treat conditions such as tinnitus. Give it a try for yourself, and you might be surprised at how much good it does you.