It could be used ina few ways. It might refer to characters on a TV show, or in a play. You could possibly use it in a story you were writing - e.g. "The bar was always home to a rotating cast of characters."

The exact meaning will depend on the context. But the general concept of "a rotating cast of .. " means that it is not necessary for one person to always play a specific role. Different individuals may come and go and play the same role. A real life example could be a band that is set up as a rotating cast of musicians. So, drummers/guitarists/singers/etc are free to join and leave and whoever are in the current roster will perform using the band's name.