Hot spots are thought to be the volcanic manifestations of plumes of hot mantle material that originate deep within the mantle, possibly even the core-mantle boundary. When the mantle plume reaches the shallow depths of the upper mantle it begins to melt and the magma rises to the surface and forms volcanoes. These mantle plumes are thought to be essentially fixed in the mantle. Relative to the moving plates they are stationary. Thus, as plates move over a hot spot, the hot spot volcanism produces a trail of extinct and progressively older volcanoes.
A good example of a hot spot trail is the Hawaii-Emperor seamount chain shown in the picture below. The bend in the chain records a change in the direction of plate motion. Determining the age of the various seamounts also allows us to determine plate velocity and its changes through time.