Take Profit and Stop Loss are the Orders that are going to be executed once the price of the asset reaches the set point.
So you can set Take Profit order to lock in already generated profits at a specified price
and you can set Stop Loss to prevent greater losses in case the direction of the price movement is not in your favour.
Note that you can add Stop Loss/Take Profit right before opening a new deal in MT4
or later in case the deal was already opened before.