A tag is the end of a song. It's usually the best part, has the prettiest chords, and is the way you leave an audience thinking about what you've just sung.
For years, barbershoppers have sung songs with great tags, and in many cases, the songs themselves have been forgotten, but the tags have survived. Some arrangers now will just write tags, and not arrange a whole song.
Tags are a great way to get into barbershop music. They're short, sweet, and give you a taste of what barbershop music without being too difficult or long.
WHERE CAN I FIND TAGS?
There are lots of places. A simple YouTube search for barbershop tags will return you lots of examples.