I'm adding this one, because someone just asked me what it meant. And, I realized that it could be confusing for a new person!
Sometimes, we'll quote something someone above us says and then we'll write "This" as our comment.
The "this" isn't an abbreviation for anything; it's truly the word "this." Or, in other words, POD*.
* For the definition of POD, see the original post in this thread.