With the Phase Switch, you can alter the phase of the subwoofer relative to the satellites. That means changing the polarity of the bass unit.
Why would you do that?
The woofers of your studio monitors and subwoofer work in phase at lower frequencies. Within the crossover bandwidth (meaning the range of frequencies that both the subwoofer and the studio monitors produce), a cancellation can occur if the subwoofer is not positioned in line with the studio monitors and/or even asymmetrically. Depending on the distance between the woofer and the satellites, either 0° or 180° may be the better position.
Again, the best way to find the appropriate settings for your system is to try what position sounds better.