The phrase "to hit a plateau" means to reach a point where you stop making progress or your skills or progress level off and stay the same for a period of time. Full definition
Basically, it is like hitting a plateau in your exercise regime where your body gets used to your normal workout that you stop making any visible progress. (metaboliccooking.site)
If you don't help kids develop broad content knowledge, especially in elementary school, they will eventually hit a plateau with their reading ability. (educationnext.org)
It is the principle that changing your routines keeps your body guessing, therefore reducing the chance of hitting a plateau. (nutritiouseats.com)