Eh, you don’t need to see any of the previous movies. Here’s a primer for the important stuff going on in this film:
[spoiler]There’s this big purple dude named Thanos who has this idea that the universe is overpopulated, and wants to do something about it. He gets a bunch of these magic stones, puts them in a glove, and uses them to erase half of all life in the universe, including most of the superheroes. The ones who survive are:
Steve Rogers/Captain America, who was frozen in ice at the end of WWII, and thawed out in the 21st century.
Tony Stark/Iron Man, who’s basically Elon Musk but cool, and has a fancy suit he invented that’s full of lasers. Steve and Tony used to be besties, but had a falling out.
James Rhodes/War Machine, Tony’s other bestie. Tony made him his own suit full of lasers.
Thor, Viking god of thunder.
Bruce Banner, who turns into the uncontrollable rage monster The Hulk when he gets mad.
Natasha Romanov/Black Widow, a former Russian assassin. (Doesn’t have a Russian accent.)
Clint Barton/Hawkeye, an American Black Ops type who uses a bow and arrows. Used to be Black Widow’s partner.
Carol Danvers/Captain Marvel, a human who was exposed to magic space radiation and now can fly and punch space ships until they explode. Hasn’t been on Earth pretty much at all since the late '80s.
Rocket, a genetically modified raccoon from outer space.
Nebula, Thanos’ adopted cyborg daughter who has recently switched sides to be a good guy.
Scott Lang/Ant-Man, a thief who stole a suit that lets him grow or shrink, and then started dating the daughter of the guy who invented the suit.
Of the heroes who are dead at the beginning of Endgame, the only one you need to know about is Doctor Stephen Strange, a wizard. He used to have one of the magic stones Thanos used to kill half of everybody, and used it to look into the future to find the one reality that would lead, eventually, to Thanos losing. Apparently, letting Thanos win in the previous movie was part of this strategy.
At the beginning of the movie, Iron Man and Nebula are marooned on a derelict spaceship. Scott Lang is stuck in an alternate dimension called the “Quantum Realm,” and has missed the whole Thanos thing entirely. Captain Marvel is helping refugees in a different galaxy, and also missed the whole Thanos thing entirely. Everyone else is on Earth, or dead.[/spoiler]
A ton of the new movie consists of clever callbacks to events that happened in the other 21 films in the series. You won’t get most of those jokes, and the big emotional beats probably won’t land, but you should be able to figure out most of what’s going on plot-wise from context.