In short, it's an envelope filter.
Here's the blurb:
Envelope Filter — A colourful, resonant filter that follows and responds to your picking dynamics.
Step Filter — A groovy, arpeggiated filter with fully adjustable steps.
Manual Filter — Great for static tone shaping or sweep-able, wah-like effects using an expression pedal.
Each of these modes offers 4 filter types: low-pass, band-pass, high-pass and peaking. Sweep range and resonance width are also adjustable, and on top of all of these, a dedicated mix knob allows blending of the wet and dry signals. This gives you total control of how much filtering effect is imparted on your guitar, bass or synth signal. Smart!