cell membranes will let some things pass through them, this means they are

Cell membranes that let some things pass through them are said to be selectively permeable or semipermeable. This means they allow certain molecules or ions to cross while blocking others, based on factors like size, charge, polarity, and solubility. This selective permeability is essential for regulating what enters and leaves the cell, maintaining the internal environment distinct from the external one.