Hypnosis
is a common human experience. You likely have not defined as hypnotic trance the experience of day dreaming, or being oblivious to the people and activity around you while captivated by a television program, or driving on "auto pilot" until arriving at your destination, or a child's active play with an imaginary friend. There are many other examples of natural trance states in which automatic functions temporarily take place secondary to unconscious activity. Hypnosis is simply a way to use the trance experience you already have, but in a very focused and goal directed fashion.
Whenever you are functioning in the automatic mode (driving, common routines, listening to something you know well, jogging, etc.), it is quite easy to drift from an alert state into an altered state of consciousness (trance). Various levels of consciousness range from alert to light trance (day dreaming) to moderate trance to deep trance to sleep. Responsiveness to hypnotic suggestion at various levels of consciousness varies greatly from person to person.