It's a way to get bitcoins on autopilot. You must have an extremely fast GPU in order to even get maybe one bitcoin a month. It's getting harder and harder to mine bitcoins do to people buying ASIC miners. Basiccly your GPU is solving Hash codes, once you solved one you have solved a "block". A block gives you a certain amount of bitcoins when its solved. Unfortunetly it is so difficult to sovle one of these now unless you have a fast miner you cant expect to get anywhere at home. Look into Litecoins. Hope this helped, its a very complex thing and im not even sure if i understand it all. Look into bitcoin mining pools if you want to start mining,