The site is very simple. You test your vocabulary and for every correct answer you provide they donate 20 grains of rice to the UN.
From the “about” page:
FreeRice has two goals:
1. Provide English vocabulary to everyone for free.
2. Help end world hunger by providing rice to hungry people for free.
This is made possible by the sponsors who advertise on this site
Some of the words are quite easy, while others are a bit “off the wall”, so it can be quite challenging.