In case you needed more evidence that the Marvel universe is moving into the multiverse, the Disney+ miniseries Loki is literally centered on the Time Variance Authority, keepers of timelines in all universes. One of the core concepts is a "Nexus," which first appears in WandaVision. Now, we're finally getting a better explanation about what a Nexus is and why we definitely should care.
As Loki explains, a Nexus Event is defined by the TVA as a moment in the "sacred timeline" — aka the master timeline that is supposed to happen in every universe — where an alternate branch is created that goes against what is "supposed" to happen. The instigators of these events are called variants, and, apparently, Loki himself is one such variant. This version of Loki is the one we meet in Avengers: Endgame during the time heist, where, instead of being captured by the Avengers and sent back to Asgard, he grabs the Tesseract in a moment of chaos and wormholes away. That moment is a Nexus Event, since it creates a new branch of the timeline that happens differently than the original.
Nexus Events are one thing, but what about Nexus Beings? Those aren't directly addressed in Loki (at least not yet), but they've been on our radar since WandaVision. One of that show's symbolism-laden "commercials" features a pharmaceutical ad for a drug called Nexus. At the time, it seemed like a hint about how the Westview world "revolved" around Wanda, but it's also a reference to something from the comics. There, Wanda is a Nexus Being, a being (of which there is supposed to be one per reality) with particular ties to the multiverse who has enough power to influence probabilities and the future.
On screen, we haven't heard much about Nexus Beings, but it seems pretty likely we will at some point. Given that we seem to be headed toward a conflict involving the multiverse, the very idea of a Nexus will probably be at the center, and we're definitely intrigued to learn more!