Comb Filter hollows out the sound by carving out frequencies at each multiple of the base frequency, like the teeth of a comb.
The Comb Filter will mix the signal with a delayed version of itself, creating a filter with repeated troughs and peaks across the spectrum. It's also possible to process left and right differently in a way that not only gives a wide stereo effect, but also collapses right back to the original signal when mixed to mono:
Comb Filter shines on it's own, but it was primarily created to power up the different "Snapin Hosts" Kilohearts offer. These are bigger effects units that utilize all the available snapins and give you a really fun and creative workflow for combining them in any way you can think of. So have a look at Snap Heap and Multipass right now.
Comb Filter is also available as a Rack Extension for Reason over at the Propellerhead shop.