It involves "sleep-training" children to self-soothe by allowing the child to cry for a predetermined amount of time at intervals before receiving external comfort.
The "Cry It Out" (CIO) approach can be traced back to the book The Care and Feeding of Children written by Emmett Holt in 1894.
[1] CIO is any sleep-training method which allows a baby to cry for a specified period before the parent will offer comfort.
Ferber does not advocate simply leaving a baby to cry, but rather supports giving the baby time to learn to self-soothe, by offering comfort and support from the parent at predetermined intervals.
Ferber discusses and outlines a wide range of practices to teach an infant to sleep.