It means that things, usually water cannot pass through it, and so osmosis could not occur in that cell.
they are made of glass because it is transparent and nonpermeable so no liquids can pass through it it is hard aswell so it will not break easily
The groyne has many features that make it useful. Gyrones can be designed however a person wants it to be designed, which makes it as permeable or nonpermeable as they want.