What it is
Accrual is the rate at which the balance grows: a monthly grant, a per-hour rate, or a front-loaded annual grant with a cap. The plan document controls accrual, forfeiture, and payout. Front-loaded plans often carry a use-it-or-lose-it rule, and several states require paying accrued, unused vacation at separation.
What to do about it: Pull the balance in writing before your last day: hours, accrual rate, cap, and forfeiture date. The date-stamped balance is what you demand on the final check.
Plain-language description of US practice, not legal, tax, or financial advice. The paired guide carries the citations, dates, and thresholds; the state pages carry the local windows.