What it strengthens:
Quadriceps, hamstrings, lower back, glutes, abs
We squat to some extent every day. For example, getting up and down off the toilet, so these are definitely a functional, everyday movement worth practicing!
Stand with your feet about hip width apart (or a little wider) and toes pointed slightly to the outside. Place your hands behind your head to help you keep your chest and head up throughout or do as I do in the GIFs to the right and hold your hands together at about collar bone height. Bend your knees reaching your hips back as though reaching your butt towards a chair. Keep your stomach pulled in the entire time! Bend your knees as far as you can while keeping your knees behind and in line with your toes. A great beginning goal would be to reach a depth where your thighs are parallel to the ground. I like to touch my elbows to my knees to accomplish this. Reverse the movement to return to standing.
To make it easier:
Hold onto the back of a chair or countertop for support and/or do not bend your knees quite as far.
To make it harder:
Bend your knees more to go lower and/or do the squat slower to give your muscle fibers a chance to get tired (and stronger).