Why is the cell membrane described as semi-permeable?

The mostly consists of phospholipids which are composed of hydrophobic ( water hating ) tails and hydrophilic (water loving).

This tail prevents unwanted polar ions and molecules to pass in the cell and restricts water soluble molecules like amino acids, glucose, and more to pass out of the cell. Also the cholesterol has the same function.

So since it only allows small and non polar molecules to pass in, it is described as partially permeable or semi-permeable. Those ions and molecules restricted by the phospholipids pass in and out of the cell via membrane .