The latest trailer for Spider-Man: Far From Home established that the film's Mysterio is a hero from another universe- and a new clip from the film goes even further on that explanation.
Shown during an interview segment with Mysterio actor Jake Gyllenhaal during an appearance on Ellen, the clip (not yet officially released online) expands on the brief moment seen in the trailer where Mysterio explains his origin.
In the new clip, Mysterio explains that Spider-Man's world - the MCU - is Earth-616, while his homeworld is Earth-833.
Both of these Earths have roots in comic books. The 616 universe is, of course, the core Marvel Comics universe, so named by Alan Moore in a Marvel UK Captain Britain comic. Meanwhile, Earth-833 (in comic book continuity, at least) is the home of Spider UK - a British version of Spider-Man who also served as his world's Captain Britain.
Interestingly, in comic books, Earth-833 was destroyed by an "incursion" in the lead up to 2015/16's Secret Wars, which collapsed Marvel's multiverse.
As for Earth-616, this isn't the first time the designation has made it into the MCU - it was seen written on a chalkboard in Erik Selvig's lab in Thor: The Dark World.
Spider-Man: Far From Home is due out in North American theaters July 2.