Long-arm suspension kits create more space under the chassis so larger wheels on a Jeep can be mounted. Short control arms refer to the stock assemblies that are available with the Jeep from factory.
A long-arm suspension upgrade is only required when a Jeep owner is looking to install larger tires. When large tires are installed on a Jeep with a short-arm design, the geometry of the vehicle is displaced. This causes the tires to be angled incorrectly, which then makes the Jeep lose out on ride quality.
When long-arms are installed, they extended the reach of a control arm. This allows the tires to be mounted correctly without putting stress on the Jeep's chassis.