The Redhorn Gate is located on the Southern side of Caradhras, the great peak dominating the central Misty Mountains. The mountains are treacherous and difficult to cross, defeating first Bilbo and the dwarves in The Hobbit, and then the Fellowship in The Lord of The Rings.
The Redhorn Gate itself is the high pass connecting the lands of Eregion and Azanulbizar, accessed from the East via the Dimrill Stair. Historically the pass was used by hobbits when they first migrated West from the Northern vales of the Anduin to Eriador.
The painting shows the Fellowship as they try to cross the Misty Mountains using this pass. Severe snowstorms sent by Saruman create drifts so high that the hobbits and Gimli the dwarf cannot find a way through, and the Fellowship are forced to travel South and pass under the mountains through Moria, which proves disastrous for Gandalf.