LSL is the Linden Scripting Language. This is the scripting language used to program everything in Second Life. Advanced programmers are used to having libraries of code that can be used in several programs. No provision is made in Linden viewers to provide this feature. Emerald had a Preprocessor that provided programmers the feature. Phoenix has now added the feature.
Toy Wylie has added a section to the Second Life Wiki on what the feature is, how to setup, how it works, an explanation of the “optimized” nature of this preprocessor, and much more.
Find it here: Wiki: Phoenix LSL Preprocessor Info