As defined at the reference,

“The Peter Principle states that "in a hierarchy every employee tends to rise to his level of incompetence", meaning that employees tend to be promoted until they reach a position at which they cannot work competently. It was formulated by Dr. Laurence J. Peter and Raymond Hull in their 1969 book The Peter Principle, a humorous [1] treatise which also introduced the "salutary science of hierarchiologyâ€