These are the abstract syntax tree nodes (subclasses of SimpleNode) - the internally compiled form of a bsh script. Instances of these nodes are created by the parser to represent the bsh syntax read from a script.
The original class structure was created by JJTree but these files are *not* automatically generated any longer. They have been implemented with the real code that executes their syntax.
Most BSH nodes are referenced only through their eval() method which causes them to evaluate any child nodes and execute their behavior, returning an object to the caller.