Why is the plasma membrane called a selectively permeable membrane?

The plasma membrane is called a selectively permeable membrane because it allows certain substances to pass through while blocking others. The plasma membrane, also known as the cell membrane, is a critical component of all cells. It is composed of a double layer of phospholipids with embedded proteins. 

The membrane controls the movement of substances in and out of the cell, ensuring that only specific molecules, such as water, oxygen, and nutrients, can enter while waste products and harmful substances are kept out.  This selective permeability is essential for maintaining the cell's internal environment, allowing it to perform its functions efficiently.