Also known as "AP."
The definition of an Artist's Proof varies from company to company. For the most part, AP's are extra pieces created of a statue that are not part of the normal edition size. These are created to cover returns and breakage and to give as comps to sculptors, painters, companies, friends, family, etc.
Some companies use AP's as selling tools. A normal edition might have 300 pieces, but they might number 25 of their Artist's Proofs and sell them for slightly more. Sometimes, variations in paint or sculpt are made to differentiate the AP's from the regular editions.
The discussion of value of AP's is a many-sided one. Some AP's have fetched higher prices on eBay, but some suffer from their lack of numbering. The price issue falls on the head of the individual collector and how much they're willing to pay for it.