It's fine
What it really means
Depends heavily on tone. Said brightly, it means things really are fine. Said flatly or with a sigh, it often means "it is not fine at all, but I do not want to talk about it". "Fine" is one of the most loaded words in British English.
Context, tips & common mistakes
"It's fine" is the most tone-dependent sentence in Britain. Delivered lightly, it means exactly what it says. Delivered flat, clipped, or — the giveaway — repeated ("honestly, it's fine"), it frequently means the opposite: something is wrong, the speaker has decided not to discuss it, and the conversation is closed for now. 'Fine' is where British feelings go to hide.
Navigation: if you suspect un-fineness and the relationship is close, one gentle later-on opening ("you seemed a bit quiet earlier — everything okay?") is right; interrogating the word in the moment is not. If you caused the issue, a small concrete make-good — the coffee, the corrected email, the washed-up dishes — outperforms further apology. And calibrate your own usage: answering real offers of help with reflexive "it's fine" is how newcomers accidentally refuse things they wanted.