An expletive is any word that fills out (
ex-ple-) an utterance without contributing anything significant to its meaning. As well as the
superfluous rhetorical phrases mentioned in the previous write-up, this includes
hesitation noises such as
um, er, filler words and expressions like
well, like, I mean, sort of, and grammatically necessary but empty words like
it, there.
It's not the word as such but the context of use. In It's raining, the it doesn't refer to anything that's raining, and in There's no point complaining the there doesn't point to a place where the point of complaining is. So these are both expletive uses, in the grammatical sense. Contrast with It's a reindeer and There's the money, where the words have their full sense and are not expletive.
In You write well and I like monkeys and I mean what I say the words have full, non-expletive value, as opposed to expletive Well I suppose I could.
Swear words are often used expletively, as in "So 'e fuckin' walks into the room, right". But it's not expletive (in the sense current in linguistics) in "I'm fucking your daughter". The idea that an expletive is something like a swear word comes from the Nixon tapes, where the transcript continually included the phrase "Expletive Deleted".