Seems to me it's a misapplication of a chess term. People are making it sound like this movie is the grand finale, all-in, winner-take-all showdown. I always thought the endgame was the most boring, mechanical part of chess. The fun part has already happened.
If a player has an advantage, even a single pawn, he/she protects it and trades all the other pieces off the board to keep the pawn and preserve the win. The player who is behind resists and stalls to the extent possible, hoping the other player makes a mistake and gives back the advantage. If the sides are even, both players usually play it safe to salvage a draw and try again next game.
Doesn't seem like a very exciting finish for an Avengers movie...