Bird dogs are not like crunches: They’re not agonising to do and you might wonder if you’re working out at all. But you are. You work on those little stabiliser muscles in your core to help you move more powerfully, and teach yourself to start using your hips more and your lower back less, which helps reduce lower back pain.
They look easy to do, but most people butcher bird dogs and further reinforce the things we want to improve (using less lower back to move). The video by Luke Sniewski goes over the exercise’s proper form and nuances. Start on all fours in a quadruped position; knees should be under your hips, hands under your shoulders and your pelvis “tucked in” (not pointed upward). Keep the entire length of your spine straight. Slowly raise one arm and the opposite leg straight out, while keeping your whole body still. Imagine there’s a broomstick on the back of your head and back, so don’t let it fall.
The goal is to resist the forces that make you unsteady while you do this. If this is hard, good, focus on technique and your posture. The article linked below goes over a bunch of variations on benches, uneven surfaces, different elevations and so on for people that find properly done bird dogs to be too easy.