Snoke is the mysterious leader of the First Order who appears in giant hologram form in The Rise of Skywalker. He is a powerful force user who is skilled in the Dark Side. His face is twisted and scarred and it appears he’s been through a battle or maybe some freak accident.
It is never explained how he became so powerful and how or why he assembled the First Order. He’s also able to seduce Ben Solo to the Dark Side of the Force which seems surprising since he’s just this bad guy who has basically come out of nowhere and has no previous connections to the Solo or Skywalker families.
His backstory needs to be fleshed out in The Rise of Skywalker. With one of the more recent TV spots, it looks like that indeed is going to happen. If you haven’t seen it or are avoiding any type of spoilers, it’s probably best to stop reading.
Okay, the warning is over.
It shows Kylo Ren descending into some horrifying crypt looking cave. Then Palpatine’s voice is heard telling him that he’s been every voice Kylo has ever heard in his head. As he tells him this, the voice changes from Palpatine, to Snoke, to Vader. This at least tells us that Palpatine is connected to Snoke. To what extent remains a mystery.
Was Snoke an apprentice of Palpatine’s? Was he just a vessel for Palpatine’s spirit? Was the black ring on Snoke’s finger a Sith artifact that Palpatine possessed? This will likely be explained in detail in Episode IX, but for now, it does appear Darth Sidious was behind Snoke all along.