Composure in football is one's ability to perform well under pressure. In big games players can either rise to the occasion and play well or play poorly under pressure.
You can get a sense of if a player is composed if they play well in big games under the bright lights when their teams need them. Some players show their composure by constantly scoring penalty kicks when the game is on the line. Defenders and goalkeepers with composure make crucial saves or tackles that give their team a fighting chance. Composure is something that often time comes with experience and character so usually captains are the most composed players on the pitch.