Ok, here we go...
Captain America: Civil War is a clever, tense and often emotional, rollercoaster ride of a film; charting the struggle within the Avengers as Rogers & Co. have the fallout from their actions laid out before them. Consequence is the over-riding theme of the film.
The chain of events from the previous films, added to the MCU's own version of the "Stamford Incident", leads to the Sokovia Accords, a piece of legislation that leads to a schism within the group. Stark is pro-registration and the belief that the Avengers need to be accountable to a higher power, whilst Cap is still very much for personal freedom and accountability, and fears that a higher power may send the team on missions that serve the ulterior motives of their new handlers.