Most volcanoes are found near subduction zones and mid-ocean ridges. This explains why the map of volcanoes on Earth matches so well the map of tectonic plates. On the other hand, some volcanoes are located in remote places like in the middle of the Pacific plate (Hawaii), thousands of kilometres away from the nearest plate boundary. This kind of volcano is very special: it is the consequence of a hotspot.
A hotspot is a mantle plume, seemingly coming from the outer core of our planet. It is a flow of hot and less dense material that crosses the solid mantle and breaks through the crust. This process creates intense and very localised volcanism at the surface. As the tectonic plates are continuously moving, hotspots leave a series of volcanic mounts at the surface of the Earth. A good example is the Hawaiian-Emperor seamount chain, a chain of ancient volcanoes created by the hotspot currently below Hawaii, which stretches over 5800km from Hawaii to Russia.