If a quasar is an extreme emission of light, what is actually creating the light?
Super heated matter generates the light emitted by a quasar.
Most galaxies are thought to have a supermassive black hole at their centre. In the case of young galaxies there is also a lot of gas and dust at their centre. This material spirals into the black hole creating what is called an accretion disc.
It takes a surprisingly long time for material to fall into a black hole. In fact to an outside observer, nothing actually ever reaches the event horizon, it just gets red shifted into invisibility.
As more and more material falls into the accretion disc it gets super heated by friction and gravitational forces to the point when it emits vast amounts of energy.
This super heated mater is what generates the light and makes the galactic core a quasar.