The settings on the back will vary in use, and often are not needed. The lowpass setting is used in the case that you have your subwoofer connected as I mentioned above. What this does is limits which frequencies the subwoofer will reproduce. You would typically set this at the frequency that is the bottom limit of your front left and right speakers.
The phase setting is used if the unit sounds like it is firing out of sync with the rest of your system. This can occur if you have placed the subwoofer in a different part of the room than the front speakers. If this occurs, you would flip the phase to 180 to correct the issue.
Finally, the level setting on the back of the subwoofer allows you to adjust the subwoofers output in relation to the rest of the system. This way you can increase or decrease the subwoofer's output to taste.