It’s a vocabulary test. For every word you get right, FreeRice donates 10 grains of rice to a hungry persons around the world. The rice is distributed by the United Nations World Food Program (WFP).
FreeRice automatically adjusts to your level of vocabulary. It starts by giving you words at different levels of difficulty and then, based on how you do, assigns you an approximate starting level. You then determine a more exact level for yourself as you play. When you get a word wrong, you go to an easier level.
There are 50 levels in all, but it is rare for people to get above level 48.