The core is more than just a six pack. Yes, this looks good at the beach, but core is so much more useful than that. Core strength promotes proper spinal alignment and stability (posture). Without a strong core (good balance), it will effect sports and physical activity negatively. A simple way to think of exercises for the core is anything that engages the abdominal and back muscles. The core is the muscles of the spine, trunk, and pelvis. These muscles are the erector spinae (lower back), external and internal oblique, transverse abdominis, and rectus abdominis.
Core stability is the coordination of the major joints and muscles of the pelvis, spine, and extremities. The importance of core stability is that it improves neuromuscular performance, improves body control and stability, improves balance, and helps transfer more power to the extremities. For most people, the most important benefit of core exercises is that it helps prevent overuse injuries of the extremities and core (lower back pain is one example). So when you are working out, make sure you work your core. Instead of just doing regular sit-ups (which is still a good exercise), try doing a plank as well.
The plank is when you are in a pushup position, but you are resting on your forearms with your elbows bent 90 degrees. Try to hold your body in a straight line by engaging the core. If you are unsure if you are keeping your body parallel to the floor, you should ask someone to watch or execute the exercise looking in the mirror. You should perform this exercise for time. Start out slow with 15-20 seconds and gradually increase the time. If this is too challenging at first, you can perform the exercise on your knees. Try this excellent movement during your next work out and reap all the benefits of a strong core. Enjoy!