If you can hold a plank for ages and crack walnuts with infinite sit-ups, you've got good core strength. But a strong core is more than brute-forcing your way through ab exercises. Your core needs stability to protect you from injury. In other words, you need core strength and core stability.
Image by Ben Sutherland.
Proper core stability helps you maintain a good spinal position and avoid potentially hurting yourself when you deadlift, squat, throw someone in a wrestling match or do another sort of athletic manoeuvre. Try this simple exercise (called a bird dog) to test your core stability. No cheating!
So two major questions here: Can you keep your spine straight (neutral) while you alternate arms and legs? Can you also keep your pelvis from wobbling throughout the exercise? The article linked below includes a few other tests and conditions that you can try. It also goes over exercises that can improve your core stability, such as bird dogs (the exercise above), dead bugs and half-kneeling resisted chops.
Unlike exercises like planks, sit-ups and ab rollouts, which improve core strength and are hard, core stability exercises seem "easy", but the point is to master control of those small stabiliser muscles in your abs.
You Don't Need Core Stability or Core Strength (You Need Both!) [Tony Gentilcore]