It's a volume pedal, or you can use it to pan between two outputs and inputs. The Next Step series pedals use a gravity sensor rather than a pot or optical sensor, so they're hard to break etc. It seems like a weird idea but it works surprisingly well.

Model: Electro-Harmonix Next Step Pan 
Made: USA, recent model
Serial: obscured/missing
Type: volume/expression
Power Input: standard 9v DC centre-negative (supply not included)
Accessories: none

Condition notes: used, no major marks. Tested and fully functional.
Overall: very good condition (8/10)