Why are cell membranes said to have selective permeability?

Selective permeability refers to the fact that membranes allow only certain substances to pass through and gain entry into the cell. Other materials are not allowed access to the cell, and they are unable to permeate through the membrane.
The phospholipid bilayer (consisting of two layers of phospholipids) enables the membrane to be selectively permeable. In short, the bilayer consists of phospholipids with hydrophobic and hydrophilic ends. While the hydrophilic ends point both toward the cell interior and the external environment, the hydrophobic ends point towards each other. This enables the cell membrane to only allow lipid-soluble molecules to pass through while blocking the non-lipid-soluble molecules. For example, charged ions and polar molecules are repelled, while lipids and small non-polar molecules can pass through the membrane.
By regulating the transport of material in the cell, the membrane maintains homeostasis.
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