I'm just going to make this simple...a jet is a hollow needle in the carb that flows air or fuel into the engine as a mixture. If you modify ur banshee with soemthing that will affect airflow (ex..filter, pipes, etc...) It will allow more air into the engine but the same amount of fuel, therefore causing a lean condition. If you put larger jets in more fuel can flow through therefore even the mixture out. STOICHIOMETRIC is about 14.7 parts of air to one part fuel, but peak performance is attained at around 12.5:1, although that can be dangerous because of detonation, preignition, etc.... If you followed what I just said (I didn't so don't worry...I blabber alot) then you should be set to go, but for a longer explanation benbb's jetting faq link is above.
P.S. - Why can't someone manufacture a fuel pressure regulator/ pump setup to eliminate most jetting problems? I gaurantee they'd make money and also take some of the fear out of modding a quad, especially if your lazy and/or don't have a downtime quad.