Hoth is the sixth planet in the remote system of the same name. It is a world of snow and ice, surrounded by numerous moons. Small meteorites that have wandered away from a nearby asteroid field constantly pelt Hoth's icy surface.
Following their victory at the Battle of Yavin, the core group of Rebel Alliance heroes had to abandon their headquarters at Yavin 4. They eventually set up a new outpost, Echo Base, on the ice planet. The temperature extremes required a great deal of effort from Alliance technicians to keep all their vital supplies from freezing.
An Imperial probe droid -- one of thousands dispatched throughout the galaxy -- eventually discovered Hoth. The Rebels were fortunate enough to learn of the droid, and began evacuation procedures immediately. The Empire arrived in the Hoth system, and invaded in a historic confrontation known as the Battle of Hoth.