what exactly does a zobel network do?

hi, i'd appreciate a "cliffs notes" version of the functions, as well as the pros and cons of a zobel. i'm just too busy to research the topic thoroughly, and would like to experiment with my current bookshelf speakers. i have read that it acts as some type of filter in the ultrasonic range, and that it also helps in terms of the overall impedance of the speaker. how does one go about deciding on the values for the resistor and capacitor? what color should they be? (ok, that was an impression of my 3 y.o., but the apple doesn't fall far from the tree :-)

thanks in advance!