jabuan is right. To make a long story short these tweaks are done when you need to optimize air/fuel mixture either because of enhancements to the engine (slip-on, full system, big bore kit, etc.) or long-term maintenance. I'm sure someone else will be willing to give a more explicit explanation.
Yep, simple. When you make a change to your bike (most often a change to the exhaust system) that either makes your bike run richer or leaner you'll normally want to either re-map an F.I. bike or rejet a carbed bike.(so it doesn't run too rich or too lean.. duh /images/icons/smile.gif)
If you need to rejet then you can buy a jet kit. Factory, Ivan's and DynoJet make jet kits for a wide variety of bikes.
If you need to remap then you'll need a power commander so you can download new maps into the bike's computer. Some bikes like Aprilia's require you to change the EEPROM chip as well although maybe someone else can elaborate as to what all is contained in the EEPROM.
Here is a diagram of a VERY simple carburetor. The items 20 - 23 are pertinent to jetting. In most motorcycle carbs there are main jets, slow jets and needle valves to adjust. In each case, using larger or smaller jets or raising or lowering the needle varies the amount of fuel introduced into the combustion chamber. In the case of fuel injection, a computer controls an electronic "valve" or injector that sprays fuel directly into the intake stream.