A return in football is a special teams play in which one team kicks the ball, either on a kickoff, punt, or field goal, and the other team returns the ball with a returner. The team kicking the ball is referred to as the kicking team, though they are technically the offensive team. The team returning the ball is referred to as the receiving or return team, though they are technically the defensive team.
The objective of any play in football is always to score a touchdown, though that is very unlikely on return plays. Thus, the goal of a returner on a return play is to establish good field position for the offense that is going to be taking the field. The returner can do this by returning the ball, allowing the ball to bounce into the end zone for a touchback, or calling a fair catch, all of which you'll learn about later.
The kicking team is the team that has the ball and is performing the kickoff or punt. They try to stop the returner from giving the opposing team good field position on returns. Gunners are a specialized role for punts, whose primary goal is to reach the returner quickly to force the returner to call a fair catch or tackle them.
The receiving team is the team that catches the ball kicked by the kicking team and performs the return. The returner is a member of the receiving team. Returners can be highly specialized as punt returners or kickoff returners.