The crux of the site is a multiple choice vocabulary quiz--you'll be given a word and four choices for the definition, then asked to pick the correct one. For every word you correctly define, FreeRice will donate 10 grains of rice to the UN's End World Hunger campaign. Ten grains may not seem like much, but it adds up rather quickly; correctly define ten words, and you've donated a bowl of rice that will feed one person.
The game has 50 "levels," and dynamically scales as you progress; when you miss a few questions in a row, it dials itself back a difficulty level, and when you get a few questions right in a row, it bumps the difficulty up. According to the website, it is "very rare" for people to get past level 48. My current record stands at