What does tag-team mean?

[Jenny Lau]

What does tag-team mean? Thank you!
Here Is the context:
Vicki Huffman, in PLUS LIVING (Harold Shaw Publishers, 1989), tells about a man who loved to hunt and bought two pedigreed setters that he trained to be fine bird dogs. He kept them in a large, fenced pen in his backyard.
One morning he observed a little bulldog trotting down the alley behind his home. It saw the two dogs and squeezed under the fence. The man thought he should perhaps lock up the setters so they wouldn't hurt the little dog, but changed his mind. Maybe they would "teach that bulldog a lesson," he reasoned.
As he predicted, fur began to fly, and all of it was bulldog fur. The feisty intruder soon had enough and squeezed back under the fence to get away.
To the man's surprise, the visitor returned again the next morning. He crawled under the fence and once again took on the tag-team of setters. And like the day before, he soon quit and squeezed out of the pen.

[twostep]

working together

[Jenny Lau]

Thank you!