The Peter Principle is a concept in management that people rise to their "maximum level of incompetence", essentially meaning people get promoted to the point where they are useless/poor at their job.
How does this come about?
Generally speaking, you receive promotions by being excellent in your current role. Then, as a reward you get promoted to a new role.
But there's no promises you might be good at this new role. Oftentimes, it will have very different skills required from the previous role you had, and you might not be good at it!
Or if you are good, maybe the next promotion puts you in an even more senior position, where these skills are vastly different and now you're rubbish!
Eventually, the theory is that everyone hits their ["Peter Plateau"] (https://en.wikipedia.org/wiki/Peter_principle) where they won't be promoted again, as they aren't performing well anymore.
The Peter Principle has a lot of interesting research online, and is interesting to read different theorists all discuss it online.
Don't let the Peter Principle scare you from ever getting promoted, just be aware of each new skill you need to learn for each new role, and be concious being constantly promoted upwards isn't a sure sign of success. Sometimes you may be at "level" you most enjoy and being promoted anymore is just not enjoyable for you :)