A line of credit (LOC ) attached to your checking account with a preset borrowing limit that can be used any time your spend more than you have in your checking account. Money can be borrowed as needed, paid back and borrowed again.
A line of credit works much like a credit card account except without a plastic card.
The interest rate for a LOC is also less then a typical credit card. It is great protection for making sure expenses coming through your checking account are paid. It also saves money in overdraft/NSF fees.