en.m.wikipedia.org
The Peter principle states that a person who is competent at their job will earn promotion to a more senior position which requires different skills. If the promoted person lacks the skills required for their new role, then they will be incompetent at their new level, and so they will not be promoted again. But if they are competent at their new role, then they will be promoted again, and they will continue to be promoted until they eventually reach a level at which they are incompetent. Being incompetent, they do not qualify to be promoted again, and so remain stuck at that final level for the rest of their career (termed "Final Placement" or "Peter's Plateau").