A Side Plank is a really key core exercise. It targets the oblique abdominal muscles, helping to improve your balance and protect your spine.
This exercise is performed by lying on your side, placing your forearm along the floor with your elbow directly below your shoulders and your legs extended out in a straight line. You will then lift your hips up away from the floor while looking straight ahead. You can extend your upper arm up above you in line with your shoulder, bring your finger tips to temples or place your hand on your hip, whatever feels most comfortable for you.
If you are finding this movement difficult you can make it easier by bending your bottom leg at a right angle as this will give you better support, taking some of the pressure off your core.