Snoke was a so-called Supreme Leader of the First Order who corrupted Kylo and brought him to the dark side as his pawn. He used his Force powers to abuse both Kylo and Hux for disappointing him.
As Kylo prepared to execute Rey as Snoke ordered him to do so, Kylo simultaneously used his Force powers to turn the lightsaber of Rey as a distraction, and bisected Snoke before Kylo and Rey eliminated the Elite Praetorian Guards. Kylo took over Snoke's position as a Supreme Leader, and he did it just to corrupt Rey and for the pair to rule over the Galaxy.
It was believed Snoke was a Bigger Bad of the sequel trilogy. But in truth, he was a decoy created by Palpatine to ensure his rule.
