The notion that there is some form of human experience that is more rewarding,
more complete, or less delusive than ordinary experience.
The subject of mysticism is wide and varying, and replete with
nonsense. Even so, there is something to it, and everybody who has been
genuinely in love, or has been drunk and happy, or can recall or recognize
what it is like to be a healthy, happy and intelligent 3-year old has at least some inkling of
what the religious mystics have been talking about.
The best general introduction to the subject and the experience seems to be
William James: The Varieties of Religious Experience - to which
it should be remarked that one can have what is fairly described as 'mystical
experiences' when one is not religious at all, and that indeed the Buddha
seems to have been originally a mystic and an atheist.
Also, it should be noted that the probability is larger that one has a
genuine psychosis rather than a genuine mystical experience, if one suddenly
finds that one's experiences turn out of the ordinary in a radical way.
Indeed, one rule of thumb to sort genuine mystics from real phoneys or true
madmen is that a genuine mystic is practically succesful apart from his or her
mysticism, is not interested in teaching or dogmatizing, does not believe a
mystical experience can be put adequately or at all in words, especially not
for those who didn't share the experience (rather like describing the taste of
a mango to those who never tasted it), and refers to an experience that is
rare and not lasting. (A brief word of advice for those who are in doubt:
Consult a medical doctor, especially when your new state of consciousness is
related to the use of drugs.)