When you say that something "is fine", it means that there's not a problem with it. For example:
A: Is it OK if I open the window for a few minutes?
B: Yeah, that's fine.
"That's fine" or "I'm sure it's fine" is not very strongly positive. It only means that there's not a problem.
You should know that "(something) is fine" is different from "a fine (something)". A "fine ___" means something that is very high quality. You use this when talking about:
- fine wine
- fine dining
- fine Art
- fine linens (sheets on your bed)