No matter what you thought of X-Men: Apocalypse, odds are you sat through the credits to see if there was some kind of tease. And there was. And it's a weird one, unless you are well-versed in comic books. So if you didn't understand the implications, we're here to help! To explain, I have to ruin another big surprise in the movie so, obviously...
Early in the film, when Cyclops, Jean and Nightcrawer go to rescue Raven, Beast, Quicksilver and Moira from Stryker, they get some help from none other than Wolverine, seen in his iconic Weapon X garb. He rampages through the barracks before meeting up with Jean and heading out on his own. This is important not just because this is a big part of Wolverine's origin, but because whatever tests they did on Logan set up the end credits.
The end credit scene is back in Stryker's bunker, where Wolverine is being experimented on. A group of people walk in, take a vial of liquid labelled "Weapon X" and put it in a briefcase with a bunch of similar tubes. When the case closes, it says "Essex Corp" on it.
The Essex Corporation is the company of Nathaniel Essex, known in the comics as supervillain Mr Sinister. In the comics he's a 19th-century scientist who, with the help of Apocalypse, is given mutant powers and made basically immortal. He spends all that time experimenting on mutants, trying to build bigger and better ones, so it would make sense that he wants the Weapon X serum.
What's most interesting about this tag is Mr Sinister's long, varied comic book past has lots of potential connections to the past, present and future of the X-Men movies. First, there's the connection to Apocalypse. We see Apocalypse enhancing mutants in this movie, but Essex was born in the 19th century -- in the movies, Apocalypse was stuck under a pyramid at the time. The next movie could just say Mr Sinister is as old as Apocalypse, but that would be a big change.
Back in the comics, Sinister is also thought to have created Scott Summers (Cyclops, played by Tye Sheridan's) son, Nathan, who becomes Cable. Cable, of course, is a character we know is coming in the Deadpool sequel. Again, a connection between the son of Cyclops (who will only be in his 20s in the next film) and Deadpool's world feels like a huge leap. The characters do have a link, though, so anything is possible.
We've also heard rumours of the female Wolverine character, X-23, coming in an upcoming film. She's a clone, and has nothing to do with the Weapon X program specifically, but the movies have often gone away from the comic book origins. It seems perfectly plausible that X-23 could be made from the blood of Weapon X in a future movie.
Basically, dropping those two words, "Essex Corp", in the ending of Apocalypse is writer Simon Kinberg opening a very large, and very adaptable chapter of the X-Men universe. Sinister is a powerful villain whose backstory offers a ton of potential links to characters new and old.