It can mean nothing and everything, there are people who use this word to neighbors and there are people which the word use for the closest person there.
So it depends on a situation and also how it was said, an intonation.
Specially in US is almost everybody who they little like Dear of Lovely and I love … is not precious term as in some other country.
If he’s family, or a boyfriend, it’s a term of endearment.
If he’s a stranger, or a person of higher authority, it comes across as inappropriately familiar and/or condescending.
It’s interesting to read previous answers from men who clearly don’t mean it that way or understand that women take it that way (and not all women do, but I’d suggest not assuming that they will be pleased based on short acquaintance, just to be safe).
The same is true of words like “hun” and “honey.” You could safely assume that if she wouldn’t appreciate your putting a hand on the small of her back to “guide” her anywhere, you probably should not call her any of these things.
[Edited to add that as a salutation in written correspondence, “Dear…” is formulaic and has no such connotations. It means nothing at all, beyond not being abrupt and beginning a letter like, “Sally…” or “Hey, you!”]