What it is
Your own contributions are always 100% vested. Employer match follows a schedule: graded (typically 20% per year starting at year 2) or cliff (0% until year 3, then 100%). Leaving before the vesting date forfeits unvested employer money, which returns to the plan.
What to do about it: Get the vesting date in writing before your last day, and check service-credit rules for breaks in service. A vesting date inside the notice window can move the value of your last two weeks.
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.