Hell is a terrible state or place of pain, suffering, and misery. If you're religious, you might believe in hell as a place where bad people go after they die.
Even if you don't subscribe to a religion that includes a concept of eternal hell for evil or blasphemous people, you might use the word to describe a terrible place or situation. Someone who's suffered an agonizing ordeal might say, "I've been through hell." The phrase "when hell freezes over" basically means "never," since hell is famous for its reputation as burning with infinite fire.