Basically, you give a dollar, choose the ideas you want Double Fine to work on in a 2-3 week "jam" and you get the games/steam keys for the top 4 voted prototypes and 2 prototypes from the last time they did this.
"In November 2012, Double Fine, along with the Humble Bundle group, announced an "Amnesia Fortnight" charity drive based on the previous Amnesia Fortnight. During this, those that paid a minimum of $1 gain the opportunity to vote on 23 concept ideas. After the completion of the voting period, Double Fine will then work at developing the top four voted ideas into game prototypes that will then be available for those that purchased the bundle. In addition, the purchaser will receive the initial prototypes of Costume Quest and Happy Song (what would become Once Upon a Monster). The development of the prototypes will be documented by 2 Player Productions." - Source: Wikipedia