PlayStation Plus Premium is a tier of PlayStation Plus that includes a large catalog of classic and streaming games that can be played at no additional cost (i.e., they are "free" once you have subscribed to this service). These games are listed below, though all games from the Extra tier are included as well. There are three tiers of PlayStation Plus in total, with the "Premium" tier being the highest one. It includes all benefits of all of three tiers.

PlayStation Plus Essential: Monthly games, online multiplayer, and cloud saves
PlayStation Plus Extra: Large included game catalog
PlayStation Plus Premium: Classics and streaming games in game catalog

PlayStation Plus Premium can be joined either via the official PlayStation web site, or by purchasing a PlayStation Plus gift card from a retailer like Amazon or Playasia.