Capacitors are a curious device that’s been around since 1745. It stores a significant amount of electricity inside a container with a high energy density and the ability to release that energy slowly or very quickly. The most common example is the flash in your camera. It accumulates the low power output of the battery into itself then releases it in a one burst. The idea of a ultracapacitor is that it can store enormous amount of energy and meter that power out over time. Another plus is that capacitors are used in thousands of applications so they are well understood. Capacitors also can be used to store power for a long time which is useful in an automotive application.
There’s always a catch though, today even ultracapacitors have a poorer energy storage by weight than lithium batteries. There are advanced being made in nanotechnology that might change this in the coming years.