The Marvel Cinematic Universe has always been broken down into "phases," which carry their own specific tone and major impact on continuity. Speaking about the history of the franchise and how it built to Avengers: Endgame, Marvel Studios President Kevin Feige revealed what inspired the idea of "phases."
Speaking to the New York Times, Feige said that he "started to use phrases like 'Phase 1 of the MCU,' because I didn’t want to merely think of an Iron Man trilogy or a Thor trilogy... It all came from the comics, individual characters would occasionally come together for a mega-event limited series, in which everything would change, and then they would go back into their own comics, with the new powers or the new costumes or the psychological changes from that big adventure."
This holds true for much of the MCU. "Phase One" primarily introduced the original Avengers, "Phase Two" reflected their growing world and "Phase Three" splintered the Avengers while introducing a slew of new heroes. It also speaks to the potential finality of Avengers: Endgame and how this is also something of an ending for the franchise, even though the MCU will continue on after the upcoming Avengers film.
Directed by Anthony and Joe Russo, Avengers: Endgame stars Robert Downey Jr., Chris Evans, Mark Ruffalo, Chris Hemsworth, Scarlett Johansson, Jeremy Renner, Don Cheadle, Paul Rudd, Brie Larson, Karen Gillan, Danai Gurira, Benedict Wong, Jon Favreau and Bradley Cooper, with Gwyneth Paltrow and Josh Brolin. The film arrives April 26.