This is a football term used to loosely describe the action of a player returning the ball immediately after receiving it via kickoff, punt, or interception. In the case of a kickoff or punt, the term runback will usually apply to longer runs which result in large gains for the team returning the ball. It may be performed by any player on the team, though there are one or two players assigned to catch the ball. In the case of an interception, the runback encompasses all yardage gained after the ball was intercepted.
The official term for a runback after receiving a kick-off or a punt is "return." In the case of an interception, either return or runback may apply. The term is most often used by sportscasters, fans, and journalists when describing the game. For example, "Joe had a great runback after he intercepted the pass."