Credit utilization is the share of your available credit that you are currently using — a balance of 400 against a limit of 1,000 is 40% utilization.
It is one of the heaviest factors in most credit scores, and it is by far the fastest to change: it recalculates as soon as your statement reports, so it can move within one or two billing cycles. It also explains a counterintuitive effect — closing an unused card removes its limit from the total, which raises your utilization and can lower your score even though nothing about your borrowing changed.