It's literally, right on the money. (At least in this country.) It means, "Out of the many, one." In that context, of course, it refers to the many states joining to form one nation. I think it has significance in the acting world as well, for different reasons.
I am referring to shows where one actor plays several smaller parts, such as being part of an ensemble, or the chorus, in a musical.
I have been in a few of these. (Most recently in the stage version of Leslie Bricusse's Scrooge, wherein I played three tiny roles in addition to my main role. ) Such work is seen by some to be of lesser importance than playing one highly recognizable and important character that can be pointed to in the playbill.
Playing many different smaller characters is like getting two (or more) for the price of one. It allows you to try a whole army of different styles and techniques all within the realm of just one show. You do not often get that chance when you are playing a single role. Ensemble (or chorus) work is a terrific time for experimentation!
It requires more focus, better timing, and a higher degree of thinking on one's feet. Many times there is even potential for stealing a scene from a principle. The next small role that an audience just cannot get out of their mind after the show could be you!
So if you find yourself in a chorus in a musical, or something similar, consider the freedom that you actually have to really show what you are made of. Let your cast mates, director, and audience know that you are a jack-of-all-trades.
Take pride in such an assignment, and out of many roles, you can present one great performance.
E Pluribus Unum.