Filter is a resonant filter with 7 modes, swiftly getting rid of frequencies you don't like or enhancing frequencies you do:
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.
Filter is also available as a Rack Extension for Reason over at the Propellerhead shop.