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)