Q.E.D.

So, when we're trying to prove something - like a math problem - we have to show that it's true. Proving something is like trying to say "see, this is right and there's no doubt about it".

Q.E.D. is something people sometimes write at the end of a proof to show that they're done and they've proven what they set out to prove.

"Q.E.D" actually stands for "quod erat demonstrandum," which is a fancy Latin phrase that just means "what was to be demonstrated."

So, when someone writes Q.E.D. at the end of a proof, it's like they're saying "See! I did it! I proved it! What I set out to demonstrate has been demonstrated."