Full-backs meaning : A player who plays on the left or right side of defence. The player who wears the number 15 jersey at the start of play. The last line of defence responsible for catching punts. An offensive back whose primary jobs are to block in advance of the halfback on running plays and for the quarterback on passing plays. A position behind the main line of backs in both rugby union and rugby league. A defensive player who assists the goalie in preventing the opposing team from scoring.