from the FAQ...
If FreeRice has the rice to give, why not give it all away right now?
FreeRice is not sitting on a pile of rice―you are earning it 20 grains at a time. Here is how it works. When you play the game, advertisements appear on the bottom of your screen. The money generated by these advertisements is then used to buy the rice. So by playing, you generate the money that pays for the rice donated to hungry people.
Does FreeRice make any money from this?
No, it does not. FreeRice runs the site at no profit.