A hot spot is a hole in the upper mantle of the earth that's located beneath the ocean AND beneath the section of earth crust that makes up the ocean floor. This hole allows molten material to push through the crust and toward the ocean's surface.
Island chains like the Hawaiian Islands are formed by hot spots. Here's how.
Remember that earth crust is made up of tectonic plates, large sections of crust that slowly move from crust formation zones toward crust destruction zones. As a section of tectonic plate moves over the hot spot, the molten material builds to the surface through the plate, eventually forming a mountainous island. But eventually, the new island is carried by the plate away from the hot spot and stops growing. Another area of plate then encounters the hot spot and another island begins to form, and so on.
In fact, even now, a new Hawaiian Island is being built by a hot spot, although its top is still about 30 or 40,000 thousand years away from breaking the water's surface.