This one includes Dream Drop Distance in HD which is the game that leads into 3, 0.2 which leads into KH3 also (it was originally meant to be a part of the intro of the game, so think of it as the Ground Zeroes of KH) and a movie detailing the behind the scenes of the mobile game.
It sounds really complicated, but it's really not. You can basically get the 3 collections, play them in number order (1.5, 2.5, 2.8) and you'll be caughtup, or play KH1, KH2, Birth By Sleep, Dream Drop Distance and that covers the essentials up till 3. The games that were spin-offs they turned into movies in the first two collections. Quite a few games in the series are skippable honestly, but if you really want a number not including KH3 there's about 8/9 games all up.