What is the "skin type" on a door? Is a door with Silicone Finish "skin type" easily paintable?
Many common interior doors in the US are hollow. There is a wood frame, with a thin layer on each side which is the "door skin".  The door may be painted, or stained to let the wood grain show.

Doors that aren't hollow, often known as "solid-core" doors, may have a thin wood veneer layer of better wood on the surface, which also might be called the door skin.

The "finish" will refer to how it is painted or sealed.  Finish can sometimes describe just the appearance of how shiny it is (flat/matte, satin, semi-gloss or gloss finish) or the material or chemicals used for the finish (painted, stained, varnished, or laquered finish).

Stain brings out the natural coloring of the wood but doesn't protect it; it needs a sealer.  The sealer over the stain is often referred to as "varnish", but there are many different chemical compounds that may be used.

In this case, the silicone finish is referring to the type of sealer over the stain.  Probably something similar to Cabot waterproofing sealer.

You will NOT have good results in painting over this finish.  Silicone definitely repels other substances.

For a door that you intend to paint, look for something either "unfinished" or "primed".  Unfinished means is just the raw wood with no paint, stain, or sealer.  Primed means it has a base coat of paint primer for protection of the wood during shipping, and it is ready for you to finish painting it.  All other things being equal, go for "primed" over "unfinished" if you are going to paint it, because the primer gives you a head start.