|This article/section is a stub — probably a pile of half-sorted notes and is probably a first version, is not well-checked, so may have incorrect bits. (Feel free to ignore, or tell me)
Linguistically, expletives refer to syntactic expletives or to expletive attributives. Both are a type of filler in the syntactic sense.
Syntactic expletives are words that play in syntax but carry no meaning.
The impersonal pronoun 'it' in 'It is raining' is a dummy subject, and does not refer to anything. 'There' can also be used without referring to anything, and therefore not qualifying for a typical pronoun.
Exactly which words and which cases this includes can be argued, and is. Some would argue there is some syntactica weather-entity in "It is raining."h
Expletive attributives are words like 'bloody' in sentences like "There's nothing wrong with the bloody thing!" It acts like an adjective, but gives no semantic detail, just signals that the speaker's annoyed.
Outside of linguistics, expletives mostly refer to bad language, apparently popularized by the appearance of "[expletive deleted]" on the Nixon tapes .