Loans are non-financial contributions to your project.
When someone submits a loan through your WishList, you'll receive an email notification. This notification will send you to the 'Loans' section of your project dashboard, where you can approve or decline the loan. In addition you must also decide what you believe the monetary value of the loan is.
For example, if a DP were to make a loan towards a camera item on a Wish List, you would then contact them to figure out the details of the offer and then approve the loan from your project dashboard (which involves setting the monetary value of the loan).
Your campaign total will then increase by the amount you inputted when approving the loan.
No actual charges will occur as a result of a loan, it's just meant to reflect the value of what you've been loaned, and to apply that towards reaching your campaign's overall goal.
Please note: Loans can only ever account for up to 40% of your campaign's total goal.