What is a mortgage?
A mortgage is a loan which has collateral backing it as a security for the lender. The collateral is usually a house. That way if you end up not making your payments, the bank will have something of value to take from you to compensate since you can't pay them the loan back. This is called foreclosure, and it happens to many Americans each year.
If your house hasn't been built yet, the loan is given to you with the agreement that you will build a house and that future house will be the collateral for the loan.