QED is an acronym for a phrase in Latin meaning something like "Thus it is proven".
Traditionally, it is used to signify the end of a mathematical proof (a set of logical steps starting from agreed-upon preliminary assumptions and ending with a new and unexpected conclusion).
But it actually means nothing on its own, it is just customary to alert the reader the proof is finished. It is used to indicate the end of the proof, sort of like "Amen" indicates the end of a prayer.
On the Zone, it usually is employed where one might normally use the phrase "See, I told you so, nyah, nyah na-na nyah."