This theory was originally supported by Snoke’s theme sounding extremely similar to Plagueis’ (it plays when Palpatine first describes his old master to Anakin in Revenge of the Sith).
He’s supposed to have been killed by Palpatine in his sleep, but a master of life might not die so easily.
The theory believes that Snoke is actually Plagueis, who has remained in hiding until the fall of the Empire and is now finally making his move.
Darth Plagueis the Wise is Palpatine’s Sith master.