Thirty years after Honey I Shrunk the Kids squashed Stan Lee's initial pitch for an Ant-Man movie, Hank Pym (Michael Douglas) and Scott Lang (Paul Rudd) finally made it to the big screen as part of the Marvel Cinematic Universe. The final film is a fun superhero origin elevated by unique effects work (read our Ant-Man review) - one that continues the work of fleshing out the interconnected MCU (in some subtle and some not-so-subtle ways). Whereas the movie's mid-credits scene advances a central thread of the Ant-Man story, while also teasing future developments, the film's end credits scene is a direct link to Captain America: The Winter Soldier and Captain America: Civil War - one that casual moviegoers might not understand on first viewing.
As a result, our explanation is going to be full of SPOILERS from here on out - not just for Ant-Man, but every MCU movie that has come so far. READ NO FURTHER unless you're all caught up. Even though most of our discussion will be speculation, a lot of it could be considered FUTURE SPOILERS for Captain America: Civil War. You have been warned.
MAJOR SPOILERS FOLLOW
Post-Credits Scene - The Winter Soldier Returns
Whereas the mid-credits scene teases Hope van Dyne's (Evangeline Lilly) future as the superhero (and Avenger) Wasp, the end credits scene is a bridge between the fallout of Captain America: The Winter Soldier, the events of Ant-Man, and the conflict to come in Captain America: Civil War. In the scene, Captain America (Chris Evans) and Falcon (Anthony Mackie), arrive at a dimly lit facility where Bucky Barnes/The Winter Soldier (Sebastian Stan) is trapped - with his cybernetic arm caught in a heavy-duty machine press. Most fans know that, in the comics, Barnes eventually takes over the mantle of Captain America (a title that is not exclusive to Steve Rogers); however, given that Cap and Falcon aren't overly-friendly with Barnes in the scene, it's safe to say that The Winter Soldier depicted in the Ant-Man post-credits tease has been on the run - and has yet to embrace a place on the Avengers roster.
As a result, Captain America and Falcon discuss how to handle Barnes - with Falcon questioning whether they should inform Tony Stark. Captain America brushes the idea off, implying that even if Stark believes Barnes is alive and no longer a threat, the Iron Man is accountable to "The Accords" - part of the pro-registration movement. Without knowing exactly how Stark might react, or how his accountability to the Accords might impact his response, Captain America decides to keep Barnes a secret from his fellow-Avenger, prompting Falcon to suggest he knows "a guy," Scott Lang, that can help free the Winter Soldier.
Who You Gonna Call? Not Tony Stark
As indicated, the post-credits scene builds on the increasingly tense partnership between Tony Stark and Steve Rogers - who, ever since The Avengers, have expressed different opinions regarding the best approach to protect the world. Disagreement was mostly superficial between Stark and Rogers in the first movie - centered on whether Iron Man was capable of "making the hard play" and sacrificing his life to protect others. However, conflict between the two heroes escalated in The Avengers: Age of Ultron - after Tony's reckless pursuit of a cure-all solution to securing Earth, lead to the creation of Ultron. The closing moments of the film show that, in spite of their differences, Stark and Rogers are still attempting to achieve the same goal - but may not see eye-to-eye on the means to that end.
Even though viewers do not know exactly how much time has passed after Age of Ultron by the time Rogers reappears in the Ant-Man end credits, it's safe to assume that a lot has happened in the intervening time. Comic readers know that Captain America and Iron Man are the poster-children for opposing sides of the upcoming Civil War storyline and, based on Cap and Falcon's conversation, it sounds as though Stark has already begun working with the government (and "The Accords") to hold superhumans accountable - a move that Captain America believes could endanger powered people and, subsequently, impede the mission to protect Earth from larger external threats.
Speculation also suggests the brain-washed Winter Soldier was responsible for the "accident" that killed Tony's parents (Howard and Maria) back in the early nineties. If true, or even if Stark were simply to believe it was true, it's understandable that Captain America might be reluctant to call on his fellow Avenger - since Tony's bitterness could cloud his judgment and, instead, put Barnes in danger.