A dark, almost black herbal jelly with a bitter taste, served chilled with honey or condensed milk poured over to sweeten it. 龟 means turtle, but many versions today contain no turtle shell.
This bowl of guilinggao has condensed milk added, so it's not as bitter.
Tips
culture
A Cantonese/Guangxi herbal jelly traditionally made from powdered tortoise plastron (龟板) and 土茯苓 (Smilax glabra root), considered cooling in TCM (清热解毒). Most modern commercial versions contain no actual turtle shell and are sweetened with honey or condensed milk.