Good Morning
What it strengthens: 
Hamstrings, lower back, glutes (posterior chain)

Cassandra's Note: 
This exercise stretches and strengthens the hamstrings as it moves them through a full range of motion. This “hip hinge” movement is an important foundational move that’s very important to master for progressing to other movements in the future as well as for your overall health.

Standing up tall, place your fingertips by your ears and hold your elbows back. Maintaining the erectness of your spine (think keeping your chest up), hinge at your hips and fold your body (like a piece of paper) so your torso comes to be more parallel with the floor. Keep a light bend in the knees and the hips back for balance throughout. Your butt should feel like you're poking it out behind you (like a cat in heat) and that's good! Squeeze through your backside to straighten back up to standing and repeat.

To make easier: 
Do not bend as far and/or bend your knees a bit more.

To make harder: 
Hold something with a little weight in front of your chest or behind your head and/or bend as far as you can while maintaining proper form.