In action/motion photos sometimes it can be difficult to maintain focus on a moving subject. Things such as other people, cars or animals moving through the frame may throw off the focus. If you’re panning, then stationary objects such as trees or poles may also cause the lens to “hunt and peck.”
Sure, if you’re practiced at manually focusing the lens it’s not a problem, but autofocusing lenses have been around for decades and very few photographers nowadays remember how to do it by hand. Most sports photographers alleviate this by doing what’s known as “back-button AF” (autofocus).
In most cameras’ default settings, the shutter button does both the autofocusing and the taking of the picture. However, in many cameras there is a button on the back near where one’s right thumb rests that can be used to focus the camera. It can be selected to be dedicated for that purpose while leaving the shutter button free to fire the camera only.
It helps a lot when shooting something that’s in motion and/or when the subject is off-center. You can either focus then quickly recompose the shot or pick another focus point in the camera.
It takes a little practice to get used to it, but once you get the hang of it, it can be a very useful technique for getting those missed action shots. Check with your camera’s manual and menu settings and give it a try.