It’s an old meme that states the first 30% of every YouTube video is useless and can be skipped without consequence. It’s a constant because it doesn’t matter if the video is 3 or 300 seconds long; The first 30% can always be skipped without missing anything important.
IIRC, it started on Reddit when a user named Wadsworth wrote about it in one of the AskReddit threads about life hacks.