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Your engine needs about 14.7 parts of air , to one part fuel to run correctly. If you you do a mod to change how air your engine can flow threw it by messing with the exhaust, or changing airfilters,Etc.. You then may need to put larger jets in your carburetor to make up for the increase in air from your mods.. The jets have holes in them about the size if a paperclip, that's what the fuel is measured threw. Actually I like 13 parts air to 1 part fuel for better power, and it's safer for your engine, but EPA likes 14.7 to 1 for economy, the environment.

 

The jets are the nozzles inside the carberator that control the amount of fuel released into the airstream though the carberator. Re-Jetting is the changing of these Jets. Intake and exhaust changes, Increase and /or decrease air flow. This can require adjustment of the fuel flow to maintain proper function and performance. You change the fuel flow by increasing or decreasing the size of the Jets.

 

It means replacing the main jet in the carb and usually replacing the needle as well. The jet is a fuel metering device. It looks like a hollow brass screw with a hole running through it. When you twist the throttle the needle lifts out of the hole and allows more fuel to mix with air and as a result the engine runs faster. It is located inside the carb under the float bowl. Shadows are set up to run pretty lean (low fuel to air ratio), when you do modifications like putting on an after market exhaust or a more free flowing air filter system you want to go to a larger jet size (the hole in the brass screw is bigger). This allows more fuel to mixed with the air and avoids a too lean condition. If the condition is too lean, the engine can run too hot and valves and pistons can get ruined. Hope this made sense.
Dave