It’s literally just saying 6-7. No, I don’t get it either. My kids thought it was hilarious to watch me grapple with it, though.
Now get off my lawn, Shorty... or pick up the phone.
I'm just waiting for Sesame Street to declare 6 and 7 the numbers of the day.
A linguist goes into the meme and it's history:
* https://www.youtube.com/watch?v=laZpTO7IFtA
You not getting it is the main point of the saying: it's slang term that young folks (in-group) are using to differentiate themselves from the not-young folks (out-group).
6 7