So if you have a 401(k) you agree to have part of your paycheck go to a fund and your employer matches however much you put in up to a certain percent. It helps get around some taxes when you eventually pull the money out for retirement.

Originally it was supposed to be a supplement for pensions but pensions meant less profits because they need to be fully funded, so over the years big corporations pushed 401(k)s as an alternative to pensions because they can just push that responsibility onto us.