A psychic trance is a state of mind where psychic abilities are amplified. A psychic trance is not a single state; many different trance states can be achieved, depending on your goal and the degree of your self-control.
There are two major characteristics which define a state of trance: focus and repetition. In a trance the mind focuses by shutting of things which are unnecessary for the task at hand and can achieve a major boost this way.
A psychic trance is more suited for tasks which are essentially repetitive, for example the 'trace' routine of psychic reading or psychic healing. Non-repetitive actions disturb the rhythm of the focus and can cause the trance to end.
For a beginner it's better to start with psychic meditation and slip into psychic trance as necessary. It should come naturally as you get better at the particular action you are doing.