The cost of running the business that isn't tied to any specific job — rent, insurance, software, your truck, the time you spend quoting.
Overhead has to be covered by every job before profit starts. The classic mistake is pricing a job at cost + profit, forgetting that overhead has to come from somewhere.
Most contractors fold overhead into their labor rate (a higher hourly rate to cover both wages and the business behind them) or add it as a separate percentage in the estimate.