Vertebrate brain
Much of what the body does is not conscious. A person breathes, heart beats, intestines digest, hair grows, thoughts pop into their mind... Some of this can be noticed by a person, and some (like breathing) can be influenced to some extent. But, basically, much of the body runs on automatic, adjusted by the autonomic nervous system. The brain, too, does much of its work without a person noticing it. The unconscious mind refers to the brain activities which are seldom noticed.