The phrase "to avoid a plateau" means to prevent or stay away from reaching a point where progress or improvement levels off or becomes stagnant. Full definition
Because you're varying the stimulus you place on your muscles, it also helps avoid plateaus. (cathe.com)
The alternation between exercises is necessary if you want to avoid the plateau effect, a state in which your body gets used to your workout and stops developing from it. (everydayhealthhero.com)
Exercise regularly, in fact include some different and little tough exercises for your body to keep fighting and avoid the plateau of weight lost. (kiipfit.com)