The technical name for planks are "Prone Iso-Ab."
If you dissect that name it means that it's an abdominal exercise, done in an isometric manner in a prone position…
You've seen these before. Basically there are many small variations but the basic one is as folows:
Start in a high-push-up position and drop to your elbows. You should have a straight line from your ankle to your shoulders. Hold that position. It's that simple. There are probably countless videos on how to do planks on youtube. I will add a small note thought for everyone that most people do incorrectly. Don't lock your hands together while in this position. It causes tightness in the Lats, thus hurting your posture and causes a slumped down position in your shoulders.