A jet is a part in your carburetor that controls the amount of gasoline that is sent to the combustion chamber. The jet can be changed or drilled out to increase or decrease the amount of gas flow. Jets are particular to the brand of carburetor (i.e. Bing,Dellorto, Gurtner). Jets are essentially a small brass screw with a thick head and a hole drilled through it. The size of the jet is found on the head. I still don't know what measurement system the jet number corresponds to.
For example, say you add a performance exhaust or high flow air filter to your bike. The engine now requires more fuel to run properly, so you must replace the jet with a larger one (upjet), thus providing the fuel amount that your engine needs. An engine can run rich or lean. This is very important on a two stroke engine. If your jet allows too much fuel compared to air, your bike has a rich mixture. Running rich won't damage anything but can cause your bike to run poorly; Not making full power and can foul your spark plug. Too little fuel in the air makes a lean mixture, which is worse for a 2 stroke engine. The air/fuel mixture in a 2 stroke engine not only provides lubrication, but also cools the engine. Running lean won’t always mean a quick failure, but it can cause the engine to run poorly and can cause a piston seizure. That said, you replace the jet/s so that the engine gets a proper mixture.