A jet is one of a few ways that a carb meters the amount of fuel going into your motor. change the jet, change the amount of fuel. They are little screw in peices with holes in them.
You will have a lean or rich condition. It will surge at cruise if it's lean, or really rich. Cruise around for a few minutes after it's warm. Shut it down on the side of the road and pull a plug. It should be a nice golden brown, tan. Not black and sutty or white and flaky. Do the same thing after you blast through a few gears and shut it down. Don't slowly pull over and let it idle for either test. It will change the color of the plugs while you idle. They jets aren't hard to change. But generally not needed to change. Holley does lots of tests at the factory before they are built. Unless you are using the thing for another application other than what it was designed, they don't need much tinkering.