Jets are metered orifaces in the carburators that determine how much fuel the engine gets. In most cases, jets can be removed and replaced with ones of different size to alter what the factory set. Since the advent of emissions regulations, factory settings are usually lean. A jet kit will richen things up from stock and is advisable anytime you make any other alterations to the amount of air the engine moves - such as an improved exhaust system and/or air filter. They may include some or all of the following: new jets, shims, needles, air filter(s), timing advancer, special instructions for carb and/or air box modifications. What you get will vary depending on the bike and what the kit maker feels is the best combination for the level of tune they're selling.
Visit the various kit makers to see what they're offering and read some of the posts in here on what tohers with your bike have done.