A pocket listing is one that the real estate broker does not place into the local Multiple Listing Service (MLS). If you're a buyer looking online, you won't see it.
As a broker, I sometimes took pocket listings. The sellers didn't want their homes to be in the MLS because they just weren't ready to jump into the showings drama just yet. Sometimes, they needed time for repairs and cleanup. Sometimes, their target date was a few months into the future, but if I, as broker, found someone looking in that price range and area, I could show the home.
Most brokers took pocket listings for pretty much the same reason. Now, though, in today's hot sellers' market, some brokers are taking pocket listings so they don't have to pay a commission to a buyer's broker. The seller saves, the buyer saves and the listing broker still gets paid. MLS' don't like the practice, even though they won't say so, because it upsets the cooperative arrangement among brokers that make MLS' functional.
This article in Inman News discusses what the California Association of Realtors is doing about it. The fact is, there isn't much any association can do, because cracking down on pocket listings hints at price fixing.
My take is that--this being America and everything--people should be free to do what they want. And who doesn't want to save a few bucks?