Hot-spots are the surface expression of rising mantle plumes - mushroom-like up-wellings of unusually hot mantle. They build great volcanic piles and, when particularly pronounced, lead to huge out-pourings of lava. Hot-spots are relatively fixed in comparison to the plates through which they "burn" and so provide a reference frame for measuring plate motion. The motion recorded for plates is no recorded relative to another plate but to the underlying asthenospheric mantle. As plates move, a fixed hotspot will "burn" a chain of volcanoes. The classic example is the Hawaiian island chain and its submerged precursor volcanoes which form a chain of sea-mounts. Other classic examples are found in the south Atlantic. These chart the separation of the South American and African continents over the past 120 million years.
How we know plates move