YouTube subbing is when you take a real conversation with friends and substitute it for watching YouTube videos, usually when someone will be reminded of a YouTube video they want to show you. This inevitably triggers a response video that someone else wants to show them, effectively killing any real conversation. The two videos are likely to be completely unrelated.
Jay: "I'm so glad to see you guys again, it's been almost a year! Let's go to the bar and catch up!"
Eric: "Oh! That reminds me of this YouTube video of this guy that's totally wasted. Here I'll show you."
Corey: "Dude, did you see that one with the guy getting his ass kicked in a street fight?"
Eric: "Hold on, we'll watch that one next."
Jay: "Are you guys YouTube subbing our reunion here?"
Eric: "Oh, you totally gotta see this Subway employee freaking out next."