This control selects the frequency at which you'd like to boost or cut. For flexibility, the frequency controls span a wide range. This makes the eq good for bass players who want to adjust the low frequency part of the signal and for acoustic guitar players who may want to remove a high frequency that is prone to feedback while playing live.
The Q determines the range of frequencies which the eq affects. i.e. If you have a wide Q and you're boosting than the frequencies around the frequency selected will get boosted as well. Conversely, if you select a tight Q and are boosting you'll tend to only boost at the selected frequency and frequencies close to it, the other frequencies will remain un-touched. When looking at the eq, 'medium q' is the left switch position, 'tight q' the middle, and 'wide q' on the right.
This determines the amount of boost or cut. At 12 o'clock there is no boost or cut. At the extremes the unit will provide 15dB of boost or cut.