RTG

"“How dangerous is it?” Teddy asked.""“As long as the container’s intact, no danger at all. Even if it cracks open, he’ll be okay if the pellets inside don’t break. But if the pellets break, too, he’s a dead man.”""“Let’s hope that doesn’t happen,” Teddy said." This is a Radioisotope Thermometric Generator, which is a big box filled with plutonium-238.

It is usually buried 4KM from The Hab with a flag (green in the book, yellow in the movie) to make sure people don't go close to it and avoid getting any type of cancer (which is unlikely even if the casing was to break).

Mark used it to heat up water for a bath inside The Hab and also on his Rover trip to the Ares IV landing site in order to save energy that would have otherwise been wasted on the heating.