You don't mean that people would pay for the game before it's released do you? If so would they get some sort of compensation for their investments after the main release?
The gamers would literally be investors. Depending on the scheme they used, it could mean they invest much more than the price of the game, and at release they could earn dividends from the profits. Sorry I'm nowhere near some sort of finance guru so I'm not sure how these things work exactly, but in principle it's fairly simple.
It would probably be a very risky investment, but to wealthy gamers it might be acceptable if they know it's making a game they want possible. They might even lose their investment of 10 times the price of the game but at least they contributed.
Again, depending on the scheme, I think it was suggested that paid-up investors could have more influence on the development direction of the game. This makes sense because they have more riding on the success of the game than some random guy on the internet who mightn't even buy the game.