Here's the definition straight from Kickstarter:
Every Kickstarter project must be fully funded before its time expires or no money changes hands.
1. It's less risk for everyone. If you need $5,000, it's tough having $2,000 and a bunch of people expecting you to complete a $5,000 project.
2. It allows people to test concepts (or conditionally sell stuff) without risk. If you don't receive the support you want, you're not compelled to follow through. This is huge!
3. It motivates. If people want to see a project come to life, they're going to spread the word.