A line of credit, or a credit line, is a source of funds that a borrower can tap into any time they wish. They are a flexible type of loan, and you only need to apply for it once to get money whenever you need it. It works like a credit card in the sense that you have a maximum credit limit, and monthly fees that you need to pay. The difference is that you don't need a plastic card to charge your purchases with, because you get the actual amount in cash instead.
Credit lines are also called revolving loans where the amount that you borrow will be subject to a maximum loan amount and no fixed tenor for some credit line offers. You may be subject to a minimum payment requirement when you do take funds from your credit line, it all depends on the bank or company.
Credit lines are still loans, however. Aside from the monthly interest and other fees you might have for taking out a credit line, you might also be asked for a collateral for a secured line of credit. Unsecured lines of credit will not have collateral, but will have a higher interest rate to account for the greater risk to the lender.