Background sound: Raindrop comes with several WAV files such as an air conditioner, a summer night, and wind noise. You can set up to three of them, and adjust each of their volume's indepdently of each other. It also has some sounds that are more for meditiation, like chanting and a "voice lullaby." Let me just say that, for things such as "Electric fan", they are very realistic, and don't sound at all like it is just a looping file.
Binaural beats: I am not an expert, but here's my explanation. If your ear here's two tones that are very close to each other in frequency (with 30 hertz), sometimes, it can make a mistake when it sends the information to your brain, giving you the impression that there is a "beat." That is a binaural beat. Binaural beats are known for being very relaxing when using low frequencies.
So anyway, Raindrop allows you to set two different tone frequencies (for example, 160hz and 165hz, and it will play the two tones out of the Left and Right speakers, creating a binaural beat, which can soothe and relax you.
The nice thing about Raindrop is that it's extremely customizable. You can have several ambient sounds going, but also some binaural beats as well. Not only that, you can set the individual volume for all three ambient sounds and all three binaural beats. You can also "automize" Raindrop by adding a "starting silence", a fade in/out time, and a "Stop after ___ seconds" with the option of shutting down after it stops.
Overall, Raindrop is an adorable little program. It's nice for someone who wants background noise, without the background noise-causer (like a fan), or for someone who just wants to relax. It's written in Borland Delphi, with the source being freely available, and it is portable to boot. The program is very tiny, at only 400kb, but the WAV files do add on to that, to about 20mb total. However, if you wanted it just as a binaural beats generator, or just wanted one WAV and trashed the rest, you could shrink it down to 1mb. It uses around 1-2mb of RAM when being run, which means that it is very good at creating background ambience in the background of your PC.