Utilization refers to your available credit (credit limit of individual accounts and the sum of all credit limits) and used credit.

It refers to balances, not using the account with any frequency. The statement balances of accounts generally get reported along with other account details including your credit limits.

Credit reports contain these numbers and utilization is calculated from them.

Try to use less than 10% of your available credit if you are trying to maximize your credit scores. If you are not trying to maximize your credit scores in the short term, it doesn't really matter.

Pushing up against the limit and being able to pay it off usually indicates you need a higher limit.