It's just a phrase... Like defining a radio that will cover extreme ranges...
DC meaning Direct Current which would be considered the lowest possible frequency resonated, to LIGHT meaning well "LIGHT" which would be considered the highest possible frequency resonated.
There is no such radio that does "DC to LIGHT" its just a term used to describe a radio that can cover every possible freq. range known to man. Like if I say I have a scanner that covers DC to Light, that just should be assumed that if there is a usable RF freq out there, it'll receive it. It's an assmily so to speak...