Speaking literally, the jets are little nozzles in the carbs. Different jets are different diameter nozzles. Kinda like varying the garden hose nozzle, ya know? Between those and the needles (they look like 8 penny nails) control the fuel flow. "rejetting" is a sorta generic term meaning swapping out your jets for the right size. That right size.... wow that depends. It's the age old debate. I'd heck out the performance guide, I think on the front page. You can toss in your setup (stock airbox, whichever pipes, etc) and it'll pull up everyone who has that setup listed. Then you can find someone in a similar climate/elevation, etc, and find out what jets they use.
For instance, I live about 8 feet above sea level, so I used bonnyUSA's setup as a guide when selecting jets. We both have the freak and Epco pipes.
Hope that helps! I never thought I'd be able to regurgitate all that
