Quoting from Kickstarter's policy:
"If the project you’re backing is successfully funded, your card will be charged when the project reaches its funding deadline. If the project does not reach its funding goal, your card is never charged.
Some backers may see a temporary authorization of funds at the time of pledging, before a project has been successfully funded. Authorizations generally drop after a few days, but this process can vary depending on your financial institution. If you notice an authorization that doesn’t drop, we recommend that you contact your bank for more details about their authorization procedure."