If you are lucky enough to have an employer who offers a 401k retirement plan, there are tax advantages to directing a portion of each paycheck toward it. You are even luckier still if that employer matches some or all of your contribution, because that is free money available to you simply for joining the program.
Vesting, or 401k vesting as its sometimes called, is common among such programs. It places a requirement on an employee to work a certain period of time before those matching dollars are yours. The money you put into a 401k will always be yours, no matter when you leave the company. But employers are able to add a time requirement as a way to encourage employees not to leave the company.
For more on 401k vesting, read this helpful description on The Simple Dollar.