Your stake duration is essentially a time countdown to when you will be able to unstake your IOTX. When the duration hits 0, your deposit is no longer locked.
Next, by turning stake-lock ON, this “pauses” your duration countdown until you decide to turn it OFF and the countdown resumes. The benefit of stake-lock is that you earn more bonus votes/rewards while it is on. After you turn the stake-lock OFF, your stake duration will resume counting down to 0. Remember, if you turn stake-lock back ON at any point during your countdown, your stake duration will restart to the original amount in which you set it. (i.e. If on day 50 of your 100 day stake duration, you choose to turn stake-lock ON. When you turn it OFF, you must now wait 100 days to unstake).
Note: Once the stake duration has expired, the unstaking process takes an additional days. After this you may withdraw your tokens to your wallet.