Cat/cow is a yoga move that unkinks and relaxes the lower back if done gently. It's usually done in "tabletop" (hands and knees), but you can also stand a couple feet from a wall or countertop. It can even be done in a chair. If leaning, make your back nice and flat like a table top and start with your head and neck in line with your spine, in a neutral position. If sitting, sit up nice and straight. It actually starts with cow. Breathe in as you look up and tilt your pelvis back, pushing your belly forward and rolling your shoulders back at the same time. Then as you exhale, look down and round your back like a cat. Repeat. Move slowly and don't force anything.
Here's a video. The poses can be much less exaggerated/athletic than shown here. The breathing is at least as important as the movement.
https://www.youtube.com/watch?v=kqnua4rHVVA