Phaser will filter the input signal, creating a series of moving peaks and troughs in the audio spectrum.
Pretty much all the bad things with signal phase issues, but turned into an effect and labeled "cool":
Phaser 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.
Phaser is also available as a Rack Extension for Reason over at the Propellerhead shop.