Soluble cyanide salts are some of the deadliest poisons known to man. Potassium cyanide is better known as the stuff in World War II-era suicide pills. Hydrogen cyanide was the major component in Zyklon B, a pesticide that the Nazis used to gas millions of innocents. And all the soluble cyanide salts do roughly the same thing: They very rapidly inhibit the body's cells from being able to use oxygen. As a result, it can kill in seconds.
Still, as YouTuber Cody's Lab shows, even one of most terrifying substances on Earth is only scary at a lethal dose — which, in the case of potassium cyanide or sodium cyanide, is around 200mg. Though he's able to feel the effects of the poison on his breathing, Cody estimates he only drank about 5mg worth, nowhere near enough to kill an adult human.
To what purpose? He's certainly not advocating for his viewers to drink poison. However, sodium cyanide has uses in gold extraction, and as an amateur miner maybe he's just trying to get ahead of all the "OMG ISN'T THAT GOING TO KILL YOU!!11!!" comments in the future.