I don't know where to ask this

Can someone just explain something to me. I don't know where to ask this question, I'm just gonna ask it here. Please just leave this up until someone has explained this to me, I reallly want to understand it.

When people in the comments section on Youtube videos, or on Twitter, or probably just basically everwhere online... when people make a post that goes like this:

Nobody:
Nobody ever:
Something something: punchline

What the hell does that mean? What are they talking about? I must have some kind of mental block, I can't understand it. I sort of get that.... no, I just don't get anything, it makes no sense to me. Can someone just tell me if it's a joke from Two And A Half Men, or if you have a minute just walk me through the logic of it? It must mean something, I see people posting it all the time.

Help!