To judder is to shake rapidly, almost vibrating. If your car's engine needs a tuneup, it may judder a bit when you first start it up or stop at a traffic light.
The verb judder first appeared in the 1930s. Experts guess that its origin was imitative, approximating the sound and motion it describes, and influenced by jolt and shudder. Engines, steering wheels, and other vehicle parts are commonly said to judder. Once televisions became common, judder was also used to describe a specific choppy quality to a show being displayed in a different format than the one in which it was filmed.