The Mirror of Simple Souls is a dialogue, chiefly between Love, Reason and the Soul, describing seven stages by which a soul is brought to nothing. Its argument is that a soul wholly taken over by divine love has no will of its own left to exercise, and that such a soul has therefore passed beyond the point where the virtues govern it: it does not need them, because it can no longer want anything other than what God wants. Reason, in the dialogue, is repeatedly scandalised, and eventually dies of the shock.
Read one way this is standard apophatic teaching in a vivid form, and the annihilation it describes has close relatives in Eckhart, in the Rhineland preachers, and further off in Sufi accounts of the passing away of the self. Read another way it appears to say that an advanced soul is not bound by the church's discipline or by moral rule, which is precisely the charge later attached to the Free Spirit heresy, and the book's vocabulary was used against people accused of it for two centuries.
The book itself declines to settle the reading. It is written to be difficult, it warns off readers who will take it literally, and it distinguishes carefully between souls that are annihilated and souls that merely say they are. That distinction is easy to lose in summary, and losing it is what the condemnation did.