From their FAQ:
What is Project 10100? Project 10100 (pronounced “Project 10 to the 100th”) is a call for ideas to change the world, in the hope of helping as many people as possible.
Why is Google doing this? The short answer is that we think helping people is a good thing, and empowering people to help others is an even better thing. Here’s the long answer.
How many ideas are you funding? We have committed $10 million to fund up to five ideas selected by our advisory board.
As best I can tell you don’t need to execute the idea or even have a plan. They are paying for the idea itself. A creative thinkers dream! Except of course you likely lose control over the idea after you win your cash.
Read the full FAQ here. Deadline is October 20th.