PlayStation Plus Extra is a tier of PlayStation Plus that includes a large catalog of 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. There are three tiers of PlayStation Plus in total, with the "Extra" tier being the middle one. It includes all benefits of the Essential tier as well.

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 Extra 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.