[Docs] A little bit clarification on how apply command resolve problems

[Altiano]

On the docs, It states

If the topic already exists but its cluster state is out-of-sync, then the tool will initiate the necessary changes to bring it into compliance.

I just a little bit curious on how exactly it'll do the changes?
I mean what does is even mean to be out-of-sync.
A list of examples would be phenomenal.