Immersion is when the players say, "remember when you shoved your polearm up the rot-wampster's butt," instead of saying "remember when we were playing D&D and you rolled that nat 20?"
Or it's whenever this guy starts talking:
It occurs to me that one reason I don't like the typical actual play video is that there's zero immersion for me. Not only am I watching a table of "funny" players instead of feeling like I'm in the story, but I'm sitting behind a screen, watching a table of "funny" players. The best ones provide some post-production images, animation, and/or sound effects.
One bonus for actual plays: the players are better at keeping the dice on the table than in-person players.