The webapp is a simple online timer. You can set a time between two and 100 minutes for your meditation session. When you click start, a bell will chime, and when time's up it chimes again to bring you back into non-meditating mode.
Satorio's creators donate 10 grains of rice to a starving person through Oxfam for every minute you meditate with the timer. 10g of rice doesn't sound like a lot, but it all adds up. 1920 minutes (32 hours) of meditation between everyone who uses the site could feed a person for a day.
The site's creators say that profits from their guided meditations and other products, such as a mindfulness journal, help pay for the rice donations.
You can use the timer without creating an account and it's a simple tool that can come in useful anyway, so why not also use it for a good cause.