A plank is an isometric core-strengthening exercise where you hold your body in a straight line from head to heels, resting on your forearms and toes, engaging your core and glutes to maintain stability. To perform it correctly, lie on your stomach, place elbows under shoulders, lift your body, keeping it straight without dipping or raising your hips, and hold the position by keeping your core and glutes tight. The goal is to maintain good form, not hold for an extended period; a well-formed plank for 20-60 seconds is effective, with longer durations potentially causing harm.