Basically it gives you an English word and asks you to pick a synonym. For every word you get right, the organization donates 10 grains of rice to the UN World Food Program to help fight hunger (the rice is bought with the ad revenue from the site, so in practical terms, you're actually earning the rice by viewing the ads).
The level goes up and down according to how many you get right (one wrong = down a level, three right = up a level). Most people start at level 30 or 40 (the first several questions assign a starting level) and apparently the levels go up to 50, but it's rare for people to get above a 48. (I've gotten to 48 once or twice, but never stayed there for more than one or two words.)
I thought there was a daily limit, but today it seemed to let me keep going without hitting a limit.