1. In tag-team professional wrestling, to tag one's teammate (usually on the hand) and enter the ring to begin wrestling the opponent. Everyone thought he was done for after being smashed by that suplex, but his teammate, The Grave Digger, managed to tag in at the last second. The crowd all booed as Nancy Fury tagged in.
2. slang By extension, to take over someone else's current activity, especially to give them a reprieve. Tom's been looking after the kids all afternoon, so I'm going to go tag in and give him a break. Would you mind tagging in for a while? I need to go make a phone call really quick.
See also: tag