Money that is vested in your 401k plan is money that is yours. You may not have access to it right now, but it belongs to you.
Think of it this way; the money in your plan may come from two sources: your own contributions and your employer’s contributions. (If your employer makes such contributions, consider yourself fortunate.) Your own contributions are always vested. That means the money is always yours plus or minus any investment results. Your employer’s contributions are a different matter. OK, how do these become vested? That depends on the specifics of your 401k plan. You might have to work a number of years before the employer contributions become vested, anywhere from zero to six years.