Jets are the hollow needles in your carburator that feed exact amounts of gaz to the cylinders. Changing the jets changes the amount of gaz that's squirted into the cylinders.
This changes your air/fuel ratio modifying the mix from lean to rich. You are always hoping to have the perfect ratio for max performance. There is a lot of debate however about what the optimum ratio is, on the other hand a very lean or very rich mixture will hurt performance.
Usually rejetting involves replacing the "jets" by small increments ( a slightly smaller or larger needle ) until the bike is responding to your liking. You would do this to resolve a factory setting that is not always set for max power or whenever you modify your exhaust. As long as it's running well and you don't change any parts there is no need to rejet.