Freud thought that a large part of the mind is not accessible and is hidden completely - he called this the unconscious mind.
The conscious mind is what we are aware of and what we can remember, discuss and deal with.
Some of what is in the unconscious is repressed (something being pushed into the unconscious by the conscious mind, though not deliberate - so someone wouldn't know what secrets they were repressing) to help someone to forget or not deal with problems.
Freud thought that there was very little being done for the mentally ill. The treatments included shaking the patients to shake out demons. He wanted to do more and wondered about mental health rather than physical health. He travelled to France and learned about hypnotism.
He then rejected hypnotism because the person couldn't remember the sessions, which he believed was important. He then realised that the mind was powerful and could cause mental health problems.