A tag is a coda or ending, that is “tagged on” to the end of the song. Tags are very popular in barbershop, because they are often taught without using sheet music.
If you attend any barbershop event, “tag circles” will be found in every available corner of a hotel lobby, and singers will stay up into the wee hours teaching each other their favorite tags like jazz jam sessions or drum circles.
The Barbershop Harmony Society has a set of standard tags with sheet music and videos.