What is a "swing dancer" in Broadway musicals?

I’m guessing that they are referring pretty directly to swing dancing , the popular American dance style made famous in the 1920s and popular through the 40s, characterized by a syncopated rhythm, a step where the dancer tends to ‘swing’ out from their partner, and the use of improvisation and borrowing from other dances, and usually based on the Lindy Hop or the Jitterbug.

A youtube video showing fast-tempo swing can be found here.

Musicals typically employ the leads, who sing and also sometimes dance, but also have more minor rolls for just-singers and just-dancers- the “swing dancers” may have been background characters who had no speaking lines or singing lines, just impressive dance. This is usually done as a sort of theatrical division of labor- lead singer/actors usually can’t also dance- no one has the time to do that kind of triple-training.

A swing (dancer or not) is a cast member who is in charge of knowing many roles or “tracks” in the show. So when a lead is not performing that night, one of the understudies steps up into that role, and the swing covers all the understudy roles.
It has absolutely nothing to do with swing dancing.

What @breedmitch said. =)

@breedmitch wow. I had never heard that term.

Yes, exactly what @breedmitch said. (I actually recently looked it up after watching a special on PBS about putting LOTR on the stage, and they mentioned it.)

FYI: as I was posting, The lovely, intelligent, all knowing gailcalled was also crafting a response. I just typed faster, and my dear Maudie let me have all the glory here. I was just so excited as this was a question with which I had some experience.
(proud Equity member)