It basically allows you to play a configurable mix of background noises; e.g. Wind blowing, rain faling, white noise, train tracks.
Heck, writing this makes me want to try it tomorrow
Noise-cancelling equipment isn't as effective for human voices, so I've used low-to-high volume music to mute out my coworker's voices. This works great until early afternoon when. I. am. Just. Acoustically. Exhausted.
I'm susceptible to music and can only use it as background for 2-3 hours at a time.
The other day I came across HN user's grahamburger's suggestion to "add in some white noise on top of" using over-the-ear noise-cancelling headphones. 
So, I did some research/shopping and am going to experiment with audio tracks from "The Very Best Sound of Nature - Birds, Waves, Rain (with Forest, Creek, Wind, Thunder) [Sound for Relaxation, Meditation, Healing, Massage, Deep Sleep, Yoga]".
Also, as a result of your suggestion for Noisli, I'm going to try a background-noise generating application. Thank you for the suggestion.