When a cell is subject to a solution that is equal in water concentration as inside the cell, the movement of water via a process called osmosis will equalize and the solution is said to be isotonic to the cell. That means the amount of water entering the cell from the solution will equal the water exiting the cell; no net water movement.
It is referred to as a selective semipermeable membrane because it only allows certain things through, due to the size and shape of whatever is trying to get through.
