Snoke uses a hologram.
He uses a hologram to communicate remotely with Kylo Ren and General Hux. We even see some dust fall from the ceiling and disrupt the projection a little bit.
CANON: Alan Dean Foster's novelization says
Seated on the raised platform that was the focus of the chamber was the blue-tinted holo of Supreme Leader Snoke.
And the Star Wars: The Force Awakens Visual Dictionary says
Snoke's whereabouts are currently unknown as all his known appearances have been done via enlarged hologram, and he keeps his command centre mobile, so as to not be tracked.
This page mentions the hologram as well.
His hologram used on the Starkiller base projected him several times larger (around 25 feet) than his natural appearance, towering over Kylo Ren and General Hux.
The scene closely mirrors a similar scene in The Empire Strikes Back in which Vader and Palpatine also communicate via hologram.