My understanding of Artist Proofs, from other mediums like statues and art prints, is the Artist Proof is the first couple of an item produced, like the first 10 or so of a pin out of 250, etc, and they are ones judged for quality, design, etc. Usually in other mediums they are sold at a higher price, because you know it was the first, or it was inspected, or many other reasons.
How they enter the pin community, I have heard of them showing up on CM lanyards, or Scoop gives them out, or at events on Pin Boards, as far as I know. There might be other ways, but I do not know what they are.