How much of your available credit you are using.
Utilization is the balance reported on your cards divided by the total limit across them. It is one of the largest inputs into a credit score after payment history.
Below 30% is the usual guidance and below 10% is better. It is measured on the balance the issuer reports on the statement date, not the balance after you pay — so paying before the statement cuts reported utilization even if you always clear the card.
Carry $3,000 against a $10,000 limit and you are at 30% utilization — and at today's 20.94% average APR that balance also costs about $628 a year in interest.
See the card apr page