What's a rotating bar?

I'd say it is a bar that is rotating.

A bar-shaped object that is rotating

Care to use it in context for us?

There used to be one in Johnson Creek Wisconsin. The place was called The Gobbler. It had a real rotating bar. It's now a small theatre. Look it up. https://en.wikipedia.org/wiki/The_Gobbler

There are many so-called "revolving restaurants," probably that serve alcohol, although they actually just rotate, they don't revolve.