Bar end shifters shift the derailleur cables (front and back, depending on the shifter). They are just another way to shift gears. Shifting gears involves moving the cable that moves the derailleur that moves the chain. Bar end shifters are very straightforward and easy to use and understand. There's nothing hidden, like in integrated shifters.
Perhaps I don't understand your question. The best way to understand them is to try them out. Or if you don't want to do that, go into your friendly, local bike shop and try it out on the workstand.