This is a type of home run that occurs when two batters on the same team hit home runs one after another. If the next hitter after a back-to-back also hits a homer for a third consecutive home run, it's usually referred to as a "back-to-back-to-back", and so on if more consecutive home runs are hit by a team.
Back-to-back home runs happen fairly often during the season. It is speculated that pitchers get rattled after a home run and that this makes them more likely to pitch balls that are easier to hit, leading to a greater likelihood of additional home runs.
The event of four back-to-back homers has only occurred seven times in the history of Major League Baseball. In 1990, Ken Griffey, Sr. and Ken Griffey, Jr. became the first father-son duo to hit back-to-back home runs.