According to the Muslim scholars, it means that Allah (may He be Praised) illuminates both the heaven and the earth. Whatever light is in the heaven and earth, in this life and in the Hereafter, is originated from the Light of Allah (may He be Praised).
There are two kinds of light: (I) A created light that exists in the worldly life and in the Hereafter and in Paradise and among people i.e. the light of the moon, the sun, and stars. It also includes the light of electricity and fire. This light is created by Allah (may He be Exalted and Glorified).
(II) The second kind is not created. Rather, it is one of the Attributes of Allah (may He be Exalted and Glorified). Allah (may He be Exalted and Praised), with all His Attributes, is the Creator and any thing other than Him is created.