Student or Learner
A: Hi! I am going to marry the love of my life!
B: Hi! What is this about getting married?
I would like to know the meaning of "this".
B is more likely to say spontaneously (on meeting A), "Hey, what is this I hear about you getting married?" But the actual dialogue doesn't matter. "This" means "this story", "this news I heard", "this rumour".
In the above dialogue, it would mean "What's this talk of getting married?"; "What's this craziness you're talking about?!"
B is asking for confirmation that what he heard (this fact/story/rumour, etc.) is true, and possibly for some explanation.