The plank exercise is a body weight exercise that strengthens the core muscles. It involves holding the body in a straight line from head to toe, with the weight resting on the forearms and toes. The arms should be positioned directly beneath the shoulders, and the feet should be about shoulder-width apart. The body should be held in a straight line and should not be allowed to sag or lift up at the hips. 

To perform the plank exercise, start by lying on your stomach on a mat or other comfortable surface. Place your forearms on the ground, with your elbows bent at 90 degrees and your palms facing down. Next, lift your body off the ground, using your forearms and toes to support your weight. Hold this position for 30 seconds to one minute, depending on your fitness level. To make the exercise more challenging, you can lift one leg off the ground, hold a weight in one hand, or perform the plank on an unstable surface. 

The plank exercise is a great way to strengthen the core muscles, improve posture, and support good form during other exercises. It is also a low-impact exercise that can be performed by people of all fitness levels.