Formant Filter shapes the sound in a similar way to how the vocal tract works, leading to vowel-esque sounds. It will boost two frequencies to mimic the sounds of different vowels:
Formant 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.
Formant Filter is also available as a Rack Extension for Reason over at the Propellerhead shop.