Filler is junk film (often blank or scratched) that is simply to fill time. This is where the name comes from. It is commonly used to fill time on the soundtrack, when there is silence in the film; in this case it is specifically sound fill. Slug typically refers to both audio and video filler. 

 

With digital media filler refers to the space, not the film, since sound and video can be moved and managed separately. In this case the filler is time that is actually not filled with anything. Slug in digital media is represented by black space or a black box on the time-line of the film. Slug is both audio and video filler, though each can be created separately as well.