What is passive absorption of a fat soluble vitamin or drug?

A passive absorption of a molecule, drug or vitamin usually entails
some sort of osmosis or membrane penetration. The fat soluble drugs
are able to pass through adipose or fat membranes using simple
diffusion.

As opposed to active absorption which requires a transport mechanism
to actually move the molecules through the membrane either through
carriage by other substances or active diffusion throughmembrane
channels.