To "max out" your 401(k) means to contribute as much as you're legally allowed. For 2018, the contribution limit is $18,500 for elective deferrals. This is just the money you're choosing to put in -- it doesn't include employer matching contributions, allocations of forfeitures, or any mandatory contributions.To be clear, this is a lot of money, and you may not need to contribute this much to meet your retirement goals.For example, if you were to contribute 2018's maximum to your 401(k) for 30 years and earned 7% returns on your investments, you'd end up with a nest egg of $1.75 million before considering the effects of any matching contributions.