This is kind of like a plank, but on your side. (AKA “side plank.”)
Start by lying on your right side, with your elbow under your shoulder and your forearm in front of you. Your left foot should be on top of your right, and your left arm should be at your side.
In one solid motion, lift your hips off the floor and your arm out so your body forms a straight line. Then lower your hips close to the floor and lift them again.
Repeat for about 10 reps. Then switch sides (so your left side and arm are on the floor) and repeat.
This is a great workout for your obliques.