In the words of Boss:

The SY-1 transforms any guitar or bass into a versatile synthesizer thats fun, inspiring, and ready to play. Backed by advanced tech first developed for the SY-300, this super-cool pedal delivers a huge selection of analog-style synth sounds while occupying minimal space on your board. Whats more, all the sounds are polyphonic, allowing you to play both single notes and chords.