Captain America: The First Avenger and, at the time, Marvel viewed it as the MCU equivalent of the Cosmic Cube. It was envisioned as a source of unlimited power, with the Red Skull using the Tesseract to create advanced weapons and vehicles for Hydra.
In the aftermath of post-credits scene in The Avengers introduced Thanos, the Mad Titan, and that clearly set up the potential for an adaptation of Jim Starlin's Infinity Gauntlet comics. Marvel realized they had a number of MacGuffins in play that could be retconned as Infinity Stones. As a result, the Tesseract became the Space Stone.
It increasingly looks as though Marvel about to retcon the Tesseract yet again, granting it new powers. Thor 4.
Captain Marvel her superpowers.
If the Infinity Stones were destroyed in Avengers: Endgame, but it looks as though at least one of them is about to make a return. The Tesseract is the Infinity Stone humanity know the best, and its return may cause a lot of problems if the wrong people get their hands on it. That's especially the case if it has an expanded powerset.
