"To strengthen the core" means to improve and develop the muscles in the middle section of your body, specifically the muscles in your abdomen, lower back, and pelvis. Full definition
This will help strengthen those core muscles necessary for rolling over. (thebabycorner.com)
Spending time each day on your ball can help strengthen your core muscles and bring everything back into alignment. (mamanatural.com)
You may be familiar with the side plank as an exercise for strengthening your core, but did you know that it also strengthens the hips in the process? (verywellfit.com)