Hawaiian islands are volcanic in nature and have formed as a result of tectonic plate movements over a hot spot. The point of contact of tectonic plates is a good location for volcanoes. However, sometimes volcanoes can also form in the middle of the tectonic plate and will release magma on the sea floor. Such volcanic regions that are fed by the mantle are known as hot spot. Hawaiian islands were formed due to Hawaiian Hot spot. The location of hot spot is fixed, however the tectonic plate (Pacific plate in this case) moves over it. This results in formation of islands. Since the plate is moving in the northwest direction, the islands are older as one moves in the northwest direction. As the plate moves over the hot spot, the magma release results in formation of volcano and as the region moves away, the volcano stops erupting and gets older and older as we move away from the hotspot (in northwest direction).
The youngest formation in the Hawaiian island series is Loihi and as we move in the northwest direction, islands get older and so do the volcanoes. Currently the Hawaiian hotspot is under the Big Island and the pacific plate is moving at 5-10 cm/yr. This motion has results in the formation of the chain of islands known as Emperor Seamount.
Hope this helps.