You almost have the idea. Here's an example that might help make the idea concrete for you. You have a universe with three balls in it we'll call them ball a, ball b and ball c. Then if ball a is an active physics entity ball b and ball c cannot be active physics entities because ball a is currently an active physics entity. But sometime in your program you can turn ball a into a non physics entity then ball b and ball c can be physics entities. But only one at a time.

If you make ball b an active physics entity then ball a and ball c cannot be active physics entities until ball b is no longer an active physics entity.

This same pattern is true for ball c being an active physics entity.

So this means that one entity at a time can be an active physics entity, it doesn't matter how many physics entities you have in your program, you can only make one active at any one time.

Get it?

Also, gravity will only effect one object at a time. It doesn't matter how many objects are in your level. Only one object at a time will have gravity effect it.