A semipermeable membrane is a barrier that allows certain molecules or ions to pass through while blocking others. This selective permeability is crucial in biological systems, as it helps maintain the internal environment of cells by regulating the movement of substances like water, nutrients, and waste products. In practical applications, semipermeable membranes are used in processes such as osmosis and filtration. They play a vital role in technologies like reverse osmosis for water purification and in medical devices like dialyzers for kidney treatment, showcasing their importance in both nature and technology.