An inoperable condition is one that can't be fixed through surgery. If someone's diagnosed with inoperable cancer, it means that an operation would not be helpful.
Many kinds of cancer can be treated at least partly through surgery, but those that can't are inoperable. Another way to use this adjective is to mean "not working." If your computer is inoperable, it won't turn on, and if your fireplace is inoperable, you can't actually light a fire in it. Operable, "able to be used," comes from operate, which has a Latin root, operari, "to work." The prefix in- means "not."