The BUILTINS.ISS file is accompanying the Inno Setup Preprocessor. It is automatically included, if it exists in the compiler directory, as if the very first line of your script contained include directive. This file contains common declarations, such as special constants for using with some built-in functions, and some useful macros. The file is a regular Inno Setup Script file but mostly contains only ISPP directives.

To learn more about the functionality provided by this file please open it with the Inno Setup Compiler, it is well commented.