Membrane permeable to a liquid (solvent) and impermeable (or poorly permeable) to substances dissolved in the liquid (solute). It can be natural or synthetic.
Depending on the membrane and solute, the permeability may depend on the size, solubility, properties, or chemical composition of the solute itself.
If molecules are small enough, then it can pass through the semi-permeable membrane, because the permeability is inversely proportional to the molecule size.
For example in hemodialysis, electrolytic equilibrium and toxics' elimination are performed by subjecting the patient's blood to the dialysis physical principle. It is made to slide in a filter in which it comes in contact with a semi-permeable membrane through which pass only or at least in a greater quantity of toxic substances) that you want to remove.