Yes, an expletive is often a swear word, but more often than not it isn't. It can be any exclamation - oh! for instance, or ouch! or that hwoof sound that old people make just after they've sat down. It's a noise that's a sign of an emotional reaction rather than possessing any particular meaning of its own.
In fact, an expletive can be any bit of language used for some reason other than advancing meaning.
So, why would anyone use words that don't help get across their message?
Well, you might use an extra word if you were trying to write a poem and what you wanted to say had the wrong number of syllables. I remember once, when I was about ten, struggling to write a line of verse and ending up with something I hated. In fact it still really bothers me now: And through my mind my dreams do creep.
See? That do? Entirely there for rhythmic purposes. Doesn't mean anything.
The fussiest grammarians also count phrases such as it is as expletives. In the sentence it is bonkers to think much about this sort of stuff, for instance, the it is doesn't really mean anything, and those people who honestly believe that Latin is the best language ever invented (Latin doesn't bother with it is) have even suggested that sentence should be recast: to think much about this sort of stuff is bonkers.
Or even ******* bonkers, if you ask me.
Thing To Use Today: an expletive. This word comes from the Latin plēre to fill.