Answer: Cell membrane ( also called plasma membrane, made up of two layers of phospholipids) is a membrane surrounding the cytoplasm of all cells and separates the internal environment of cell from its external environment.
It is a semipermeable membrane, which means that it allows the entry and exit of only specific molecules ( such as small uncharged molecules, small polar molecules like Oxygen and Carbon-dioxide, small uncharged polar molecules like water). Also, it does not allow the toxic substances to enter the cell and at the same time the useful substances to leave the cell.
By doing so, it acts like a protective barrier for the cell.