Well based on the fact that this is "Production of Materials"
I would assume your talking about the Porous barrier in galvanic cells.
Basically in galvanic cells redox occur (i.e. transfer of electrons) As electrons transfer to the cathode side two things happen:
i) The cathode solution has its less active metal become displaced, chemically bonding onto the electron inundated cathode, whilst leaving anions in its solution. Cathode solution becomes negativley charged.
ii) The anode metal is displaced from the cathode into its solution. Anode solution becomes positively charged.
AS a result to maintain the neutrality a porous membrane (or salt bridge) enables the migration of ions (cations to cathode, anions to anode) to the respective electrolyte solutions. Also the ions which migrate through the porous membrane do not react with the electrolyte solution.
Note* Not all galvanic cells have porous membranes, some of which include the dry cell and the alkaline cell where the electrolyte solutions can service both electrodes.
Hope i could help