Purpose
Layer Packages (Lpks) are very useful ways of transerring groups of data from one compter to another, while keeping the data and symbology in that data-group intact.
Basic Procedure
Below is a video that walks through the basics of layer packages.
BRAT LayerPackage Walkthrough
In the event that you need to create a layer package for BRAT manually, here is a video that walks through the entire process.