An expletive is a short phrase (often as short as a single word) that is placed neatly inside an otherwise normal sentence to give the words around it extra emphasis
(or sometimes clarity).
If the important information is at the beginning of a sentence, this is where the expletive is placed.
eg. "All truth is not, indeed, of equal importance; but if little violations are allowed, every violation will in time be thought little." - Samuel Johnson
See how the word in commas makes the words around it seem more important? Expletives can be used to emphasise all sorts of things, from short phrases to particular clauses in a verbose sentence.
The most common and useful expletives are: in fact, of course, indeed, I think, without doubt, to be sure, naturally, it seems, after all, for all that, in brief, on the whole, in short, to tell the truth, in any event, clearly, I suppose, I hope, at least, assuredly, certainly, remarkably, importantly and definitely. Overuse of expletives should be avoided in formal writing, but on the whole, they form a useful tool with which to shape a sentence.
The concept, you see, is not difficult at all.
Information drawn from http://www.vanguard.edu/rharris/rhetoric.htm