Paying off Mortgage early question

DH and I are going to start paying ahead on our mortgage. I’ve heard that paying one extra mortgage payment a year, every year, can knock off 5-7 years of the mortgage. My question is; is that the whole entire mortgage we pay now (principal, interest & escrow), just the principal, or the entire principal and interest? We are paying an extra set amount each year regardless that will be higher than our monthly payment, but I was just curious about that, as well as how much truth there is to it.