It means it only lets certain things through. A paper towel is semi permeable; liquids will pass but solids wont.
What Does Semi-permeable Mean?
Semi-permeable means 'partially permeable'. The term semi-permeable is composed of two words –semi and permeable. Semi is prefix in Latin and means 'half', or 'incompletely' or 'somewhat'. Permeability refers to the quality of diffusion or seeping or penetrating. This quality is generally associated with liquids or gases. Thus, semi-permeable is used to describe something that allows the passage of some substances and blocks other or not required substances.
This term is generally used for biological membranes or synthetic membranes. For example: semi- permeable membranes. It is also used in Geology to describe rock characteristics. For example: semi-permeable rock surface. Permeable matter has pores. Semi-permeability defines the characteristic of that substance that allows the passage of small ions or molecules of liquid or gas through the pores.
This term is generally used for biological membranes or synthetic membranes. For example: semi- permeable membranes. It is also used in Geology to describe rock characteristics. For example: semi-permeable rock surface. Permeable matter has pores. Semi-permeability defines the characteristic of that substance that allows the passage of small ions or molecules of liquid or gas through the pores.
Semi-permeable, or partially permeable, means that some things can go through it and others cannot.
Partially Permeable means that things can almost go through it. They can only go through part.