In the marvel universe, fictional legislative bills are passed that required documentation of every superhero. This means that their secret identities would be known by the government, and would only be allowed to fight if deployed by government officials. The law splits the Avengers down the middle, while some are for the law and some are against it. The "leader" of the pro-registration side is Tony Stark, who sees it as a chance to make the world safer. Captain America, however, is a part of the anti-registration side, seeing it as something that isn't legal or fair. I might be wrong on a couple of things so feel free to research more, but this is overall what the issue is about. The movie is coming out in 2016.