White light is light that's made up of ALL colors.
If you have a source of white light and you want red light,
then you have to REMOVE all the other colors from the
white light somehow.
That's what a red filter does. The dye in the filter has chemicals
that absorb every color of light except red. Then, red light is the
only thing left to come out of the other side.
(If the light going into the filter had no red in it, then the filter
would appear black, because there wouldn't be any light
coming out of it at all.)