The phrase "plank position" refers to a basic exercise position where you hold your body straight like a plank, with your hands directly beneath your shoulders and your toes on the ground. Full definition
How to: Start in plank position with wrists directly under shoulders (a). (dailyburn.com)
Get into high plank position with hands on floor, arms straight, and lower your upper torso down to the ground and then push yourself back up to start position. (robinskey.com)
Try it: Get into plank position, but with hands on floor, slightly wider than shoulder width. (health.com)