Technique where you take a photo of a moving subject with a slow shutter speed; you track or follow the moving subject with your camera before and while you press the shutter. You end up with the subject in focus but with a blurry background because you are essentially breaking the keep your camera still rule on purpose. Takes a lot of practice.

This is similar to taking a photo of a subject on a carousel while you are standing in the center of the carousel. The camera (kept still) and the subject are moving in relation to the background . The subject will be in focus while the background will be blurry.
Panning a car on coast highwayPanning a bicyclist in UCB