Hypnosis is a state when your mind becomes focused on a specific idea, thought, voice or suggestion, to the exclusion of all others . It has been proved with MRI scans that your brain cannot tell the difference between what you imagine and reality so because you control what you imagine, you can very safety change habits and memories to what you desire now.
Most people feel deeply relaxed, like that lovely half awake and half asleep feeling first thing in the morning. You will hear what the therapist is saying without needing to listen. Even if you drift off to sleep briefly, your unconscious mind will still be listening. Your unconscious mind will reject suggestions that it does not agree with so it is perfectly safe.
Hypnosis is a very powerful tool that allows you to talk to your unconscious mind and change the programming that you have picked up during your life. 90% of what you do is unconscious and you are largely programmed by the age of 7. So your parent, siblings, teachers, TV etc. programmed your lives for you. Hypnosis and self hypnosis allows you to change how you live your life.
We all experience hypnosis on our own in our everyday lives. For example, when you drive to work thinking about other things and arrive without really thinking about how you got there. Your unconscious mind takes over guiding us along the route, around corners and stopping at stop lights and so on. When reading a book that you cannot put down or when watching an enthralling movie, you are focussed so intently that you are in effect, in hypnosis.
A trained hypnotherapist helps you to focus the unconscious mind so that you process things differently and as a consequence behave differently. Hypnosis is very effective to help people quit smoking, release weight, sleep better, overcome phobias and anxiety, and manage the symptoms of PTSD.
However you may want to be cautious of people claiming to be hypnotists who have only done 3-4 day hypnosis courses with no therapeutic background.