What exactly is Hist Sap? Is it a drug like Moon Sugar? Does it play a role to Argonians like Moon Sugar plays to Khajiit?

It is still a mystery. But it doesn't effect Argonians in a way to make them be tripping hard. In lore, hist sap is said to be licked (and how many times) to determine the appearance of the Argonian.

They traditionally worship the Hist, and drink Hist sap. Kinda like how a Church is to wine

Could also be a way for the argoninans to establish a link between them and the Hist, and to fully develop, or something.

it's the sap of a hist tree -_-

Baby argonians are balically just lizards, and at their coming of age they lick hist sap. the more they lick, the more merish they get. (although it also depends on their tribe and whatnot)