Not that I'm the best expert in it, but from what I understand, it's a P2P computer power exchange. Through Golem network, you can rent out your computation power (like how you rent out your storage space with Siacoin) and earn GNTs. Or you can buy computation power, as if you're renting servers from AWS.

Doing so through the browser would be even cooler; though I'm not sure if that's technically possible.