This means the same as porous. Fluids will penetrate permeable seal components. This is the reason we want dense carbon-graphite and hard seal faces or otherwise bacteria and similar substances will permeate into the faces.
It also follows that the denser the seal face, the better it can seal vacuum.
Some chemicals can permeate into the dynamic and static O-rings we use in mechanical seals and blow out the low pressure side. This is a problem with the fluid ethylene oxide.
The solution to sealing ethylene oxide is to use dual seals with a high pressure barrier fluid between the seals to prevent a pressure differential across the O-rings.