What is it meant by a partially permeable membrane?

A membrane is said to be partially permeable because it only lets certain substances through. The membrane will not allow substances through unless it has a specific component to do so. For example, simple gases like oxygen and carbon dioxide can pass through unaided but larger molecules like glucose or charged molecules like ions need proteins to help them pass

A partially permeable membrane is a membrane which only allows particular substances cross it, and not others. Some substances like water are made of molecules which are small enough to cross the membrane easily. Some molecules are too large to cross the membrane, and other need specific channels to help them cross the membrane. This allows the cell membrane (a partially permeable membrane) to control what enters and leaves the cell.

A partially permeable membrane will allow water and small, lipid soluble molecules through, but not large molecules.