Let’s say you’re primarily a vocalist, but you know your way around Shakespeare. That makes you a “singer who acts.” If you spent years perfecting your pirouettes in ballet class, but you also know how to belt, then you could be pigeon-holed as either a “dancer who sings” or a “singer who dances.” But what if you’re a master at all three—singing, acting, and dancing? Then you’re a TRIPLE THREAT.
In musical theatre jargon, being a triple threat means you can do it all. Not every performer is equally skilled at every aspect of showmanship, nor (I suppose...) do they have to be. But it helps.
Once upon a time, the leads would glide offstage during big dance numbers, leaving supporting players and the chorus to handle the choreography (think of the dream ballet in Oklahoma!, with “Dream Laurey” and “Dream Curley” taking over for their actor-singer namesakes). Many shows also had duplicate choruses, with one ensemble primarily used for singing and another for the dance numbers.
Nowadays, performers often have to be expert as singers, dancers, and actors. That’s because being an “actor who sings” or a “dancer who acts” might not be good enough to get you a job in a show. In this competitive business, anyone with a leg (or two) up on other performers is that much closer to seeing his name up in lights.
TDF Theatre Dictionary