A business line of credit works like a reserve. The lender approves a limit, and you draw what you need when you need it. You pay only on what is drawn, and most lines revolve, so repaying a draw frees that credit to use again.
Lines suit uneven needs: payroll on Friday before customers pay, Q4 inventory, a 60-day invoice from a GC. They fit poorly with a long-lived purchase you would rather pay off over years.
Example: On a $50,000 line, you draw $20,000 at an 18% annual rate. Interest for a month is about $300, and the other $30,000 costs nothing until you use it, apart from any maintenance or draw fees.
Ask about draw fees, annual fees and how each draw is repaid. Some online lines repay each draw on its own fixed weekly schedule.