Vesting
The rules for when your employer's contributions to your retirement plan become fully yours to keep if you leave the job.
Money you contribute from your own paycheck is always 100% yours. Employer contributions can vest over time.
With cliff vesting, you own nothing of the match until a set date, then all of it, and by law that date can be no later than 3 years. With graded vesting, you own a larger share each year, reaching 100% by year 6 at the latest. Check your plan's summary before you change jobs.
Example
If you have $4,000 in matching money and are 60% vested, you keep $4,000 × 60% = $2,400 when you leave.