This system means that there may be only one open position in the same instrument at the same time on your trading account:

• If there is an open position already, in case you open another position of the same direction and in the same instrument, the volume of the existing position will be enlarged accordingly;
• If you open a position of the opposite direction, the volume of the existing position will be either decreased, or closed (if the volume of the opposite position is the same) or even reversed (if the volume of the opposite position is larger than the volume of the existing one).

MT5 accounts allow you to choose whether the account will be a hedging or netting account when opening it.