I looked up “tapping the glass” online and the first two definitions I saw were both good.
From Pokerdictionary.net: “In poker, the saying ‘Tap the glass’ refers to telling weaker players at the table about their bad play, to vent, and to educate them in the process.”
And from Flopturnriver.com: “Tapping the Glass is a common term for telling the fish at your table how bad they’re playing.”
Both sites immediately stated in strong terms that tapping the glass is bad for business and in poor taste. What I want to talk about is the view from the other side of the aquarium.