Q.E.D. is an acronym for the Latin phrase quod erat demonstrandum meaning "which was to be demonstrated;" basically it's just a fancy way to show off you just logically proved something.
It is actually a transliteration of a phrase ancient Greek mathematicians placed at the end of logical proofs—a kind of stamp that says “I proved what I set out to. Evidence for the abbreviation Q.E.D. is found from the 17th century. The philosopher Spinoza famously used Q.E.D. at the end of an argument in his 1677 Ethics.
By the early 20th century, Q.E.D. branched out of math and philosophy into a more general term, kind of like “therefore,” “so it follows,” “obviously,” “thus,” “boom, there it is.” You get the idea.