Sterno is a fuel made from alcohol in a jelly form. It is used as an easy way to start fires.
In The Dark Tower, when Roland Deschain and Susannah Dean reunite and start preparing for the last leg of their journey to the Dark Tower, Susannah finds cans of Sterno, but Roland scoffs at the idea of needing something to create easy fires. However, the Sterno becomes crucial to their survival as they are stuck in the tunnels under Castle Discordia with a todash monster in pursuit. Fire and light is the only way to keep the creature at bay. The Sterno is used to light the bones they find in the tunnel. The cans of Sterno are just barely enough to get them through the tunnel and onto the Badlands.