The SCons wiki has moved to https://github.com/SCons/scons/wiki/Builder()
Builder() creates a Builder object for the specified action. In general, you should only need to add a new Builder object when you want to build a new type of file or other external target. A Builder object is a callable that encapsulates information about how to execute actions to create a target Node (file) from source Nodes (files), and how to create those dependencies for tracking.
Generally the Builder (SCons User Guide Chapter 19) should have a action or a generator (SCons User Guide Chapter 19.5.) parameter.
For a detailed parameter description look into the man pages scons-man.html Chapter Builder Objects
Examples for custom builder could be find at CustomBuilders