The funds that you invest in through your 401(k) have costs associated with running them. Your employer pays for some of the costs; other costs are passed along to you, the investor, through fees that are deducted from your 401(k) account.
Over time, high fees will eat away at the value of your account. Find out the fees you are paying; your employer is required to give you this information with your 401(k) statement.
To understand how this affects you the individual investor consider the following example: