In a mid-credits scene, Hank, Scott (Ant-Man), Hope (The Wasp) and Janet are attempting harvest quantum energy to help Ava Starr (Hannah John-Kamen), aka Ghost.
The Marvel Cinematic Universe has had issues for years with how, exactly, its timeline fits together, which is a natural issue when you're mixing together the continuity of over a dozen films into one cohesive timeline. Marvel Studios and the cast of the film are taking great pride in its shocking nature, which has lead them to make a strong effort to contain the spoilers, especially with the film releasing a month later in certain parts of the world than it is in the United States.
This could mean that Ant-Man and the Wasp ends at the exact moment that Thanos performs his infamous finger snap in Infinity War, which "killed" half the population of the universe and half the Avengers too. Therefore, questions varied from "How big can Ant-Man get", to "What is a Paul Rudd". Since they are under house arrest, they can not participate in the fight in Infinity War. Hank Pym, Evangeline Lilly and Hope van Dyne are on the outside, monitoring him. They manage to save her in precisely three days. As is the Marvel way, it does indeed, with a total of two after-credits scenes to check out after the movie "ends" (I guess we should lean into calling them stingers this time around, right?).
The first sequence is a mid-credits scene.
Not only does this shockingly boost Thanos' death toll, it also leaves Scott trapped in the quantum realm. Scott was warned to avoid any time vortexes in the Quantum Realm, and that seems like a very specific piece of information to drop on the audience.
Meanwhile, Scott is now stuck in the quantum realm until Feige only knows when. and if we've learned anything from Janet, it's that staying down there can do weird things to a person.
The Quantum Realm, also called the Microverse in the comics, is where Scott Lang, aka Ant-Man (Paul Rudd), goes when he shrinks down to a subatomic level.
"The Quantum Realm is the good news and the bad news in this movie". It may even be out of Thanos' reach, or at least his awareness.
In the meantime, Avengers 4 is slated to arrive in theaters on May 3, 2019 while Spider-Man: Far From Home will come shortly afterward on July 5, 2019.