It’s the OG exercise for good reason. Despite a push-up's simple mechanics, doing it properly is challenging and greatly helped by mastery of the plank, ahead. The basic version strengthens your shoulders, chest, and biceps, and adding a pinch between your shoulder blades at the bottom will strengthen your upper back too.
How to: Assume the plank position, with your hands directly under your shoulders, abs tight, and feet hip-width distance apart. Looking 45 degrees in front of you, lower as far to the ground as you can without letting your back dip or arch, then push back up without leaving your abs on the floor.
Variations: Hit your triceps! Keep your elbows close to your ribcage, and push your elbows back as you lower down. Alternatively, take your knees to the floor and cross your ankles, make a diamond shape with your hands on the floor, keep your shoulders away from your ears and your back straight, and lower and lift. Note that the closer your hands stay to your body, the more your tris will burn.