Here's the Catholic Encyclopedia explanation:
Quietism (
Latin quies, quietus, passivity) in the
broadest sense is the
doctrine which declares that man'shighest perfection consists in a sort of psychical self-annihilation and a consequent absorption of the
soul into the Divine Essence even during the present life. In the state of "quietude" the mind is wholly inactive; it no longer thinks or wills on its own account, but remains passive while
God acts within it. Quietism is thus generally speaking a sort of
false or exaggerated
mysticism, which under the guise of the loftiest spirituality contains
erroneous notions which, if consistently followed, would prove fatal to morality. It is fostered by
Pantheism and similar theories, and it involves peculiar notions concerning the Divine cooperation in
human acts.
In a
narrower sense Quietism designates the mystical element in the teaching of various
sects which have sprung up within the
Church, only to be cast out as
heretical. In some of these the Quietistic teaching has been the conspicuous
error, in others it has been a mere corollary of more fundamental
erroneous doctrine. Quietism finally, in the strictest acceptation of the term, is the
doctrine put forth and defended in the seventeenth century by Molinos and Petrucci. Out of their teaching developed the less radical form known as Semiquietism, whose principle advocates were
Fénelon and
Madame Guyon. All these varieties of Quietism insist with more or less emphasis on interior passivity as the essential condition of perfection; and all have been proscribed in very explicit terms by the
Church.
In its essential features Quietism is a characteristic of the
religions of
India. Both
Pantheistic Brahmanism and
Buddhism aim at a sort of self-annihilation, a state of indifference in which the
soul enjoys an imperturbable tranquillity. And the means of bringing this about is the recognition of one's identity with Brahma, the all-god, or, for the
Buddhist, the quenching of desire and the consequent attainment of Nirvana, incompletely in the presentlife, but completely after death. Among the Greeks the Quietistic tendency is represented by the
Stoics. Along with
Pantheism, which characterizes their theory of the world, they present in their
apatheia an ideal which recalls the indifference aimed at by the Oriental mystics. The wise man is he who has become independent and free from all desire. According to some of the
Stoics, the sage may indulge in the lowest kind of sensuality, so far as the body is concerned, without incurring the least defilement of his
soul. The Neoplatonists (q.v.) held that the One gives rise to the
Nous or Intellect, this to the world-soul, and this again to individual
souls. These, in consequence of their union with matter, have forgotten their Divine origin. Hence the fundamental principle of morality is the return of the
soul to its source. The supreme destiny of man and his highest
happiness consists in rising to thecontemplation of the One, not by thought but by
ecstasy (
ekstasis).
More here:
http://www.newadvent.org/cathen/12608c.htm