While some people use "trance" to describe any altered state of mind, I am going to talk about standard hypnotic trances.
Trance is the name given to any state of mind where that person has a narrow focus of attention. Typically, in the trance states that hypnotists induce, the subject has their eyes closed, is very relaxed, has a quiet mind and is aware of what is going on. Contrary to common belief, there is nothing stopping the client being able to reject any suggestion or awaken at any time -- unless the subject believes otherwise.
Whilst in a trance, the subject is able to step outside of their ordinary belief system. In therapy, this can bypass any limiting beliefs. In stage hypnosis, the hypnotist usually elicits awe and obedience. If the subject believes that they have to do what the hypnotist says... that belief functions exactly the same as if you are awake.
In addition, the placebo effect is so powerful that hypnotists often ensure that subjects realised they are or have been hypnotised. They convince the client's unconscious mind to do something the client has never experienced before, such as keeping their eyelids closed or lifting their arm up.
Depth of trance is largely irrelevant except as a last resort to:
Let's talk about your unconscious mind.