In short, it is the $66 billion resale market for art. The term has become almost synonymous with art auctions, but the term secondary market applies to any art that has been sold at least once before, and is then being sold again regardless of which sales channel. Perhaps because it sounds like secondhand car salesmen, attempts have been made to name it differently, and aftermarket crops up every now and again. However, some of the principles associated with that ilk of car salesmanship do indeed apply. The high-end part of the market is considered the most manipulated trading platforms in the world involving bidding rings (chandeliering), insider deals and planned scarcity.
The primary art market refers to the first time art is being sold from e.g. directly from the artist.