A rollover is when you move your retirement money from one tax-deferred or tax-free account to another. For example, if you move money from your ERS retirement account to an IRA, the deposit transaction is called a rollover.
When you roll over your money, you don't have to pay income taxes on the withdrawal because you will deposit your retirement money into another account. When you withdraw the money from the rollover account, you will have to pay income taxes.