Just in case you didn't know only ported or bandpass boxes are tuned.
Tuning is the DECREASE/INCREASE in BOX VOLUME / PORT SIZE(and length)
until you have the desired frequency. Tuning is done by making lots of calculations really fast... preferebly by a computer but there are tons of formulas out there that you can use. But Tuning a Box is dependent on they sub that you are using. Tuning affects the sound of the speaker system as well as how the speaker reacts and how much wattage a sub can take at a specific frequency. I think that should cover the main aspects of it.