When setting up any sort of draft system, chances are that you'll have multiple kegs, but only one CO2 tank. When you have more kegs than CO2 tanks, you'll need 1 of 2 things: a line splitter, or an air distributor.
Air Distributor
An air distributor is the way to go if you want to splice your CO2 line into multiple kegs. The reason an air distributor is better than a simple line splitter is that it will allow you to turn the air to each keg on/off individually. Air distributors will provide a lot more flexibility in adding and removing kegs from your draft system.
Line Splitter
A CO2 line splitter will be exactly that. One line in, two lines out. These are fine in a bind or at parties, but generally speaking, a multi-valve air distributor will be the way to go.