"PC" stands for "Publisher's Copy" or "Presentation Copy." Historically, they were lettered and numbered copies that were not part of the lettered or numbered sequence that were given to the author, artist, and other people related to the production of the book. When a lettered edition of, say, 26 books is produced, extra copies are produced to serve as PCs and in case one or more copies is damaged in printing or shipping. All of these leftover copies eventually get marked as PC and find their way on to the market, one way or another (usually, people who received them sell them).
The PC books are identical to the numbered and lettered books, except for the designation. To some collectors, that makes a difference---copies that are part of the sequence are considered more valuable. It used to be that PC copies were actually rarer (because you had to be involved with the production of the book somehow to get one) and thus more valuable, but that all changed about 25 years when Dark Harvest PC copies were flooding the market. There were accusations that they had intentionally overprinted and sold the books as PC editions. Ever since then, PC copies have generally been considered less valuable to obsessive collectors.
Hunter