This cheat toggles (enables and disables) immortal mode for your character. In immortal mode, your character will still takes damage but cannot die as your health will never reach 0.
If already in immortal mode, this command will disable immortal mode. If not in immortal mode, this command will enable immortal mode. Immortal mode is not the same as god mode - in immortal mode your character still takes damage but cannot die, even when health reaches 0.