When the C# compiler encounters an #if directive, followed eventually by an #endif directive, it will compile the code between the directives only if the specified symbol is defined. Unlike C and C++, you cannot assign a numeric value to a symbol; the #if statement in C# is Boolean and only tests whether the symbol has been defined or not.
A predefined (by default) symbol on the "Debug" build configuration is DEBUG. Using this symbol you can define code that will be compiled only in Debug; for example, a debug window will be shown only when needed.
The code above will print "Debugging information" and "Code that always executes" when build on Debug and will display only "Code that always executes" when build on another configuration.
You can suppress the definition of the DEBUG symbol from the project properties or by removing the DEBUG from the build argument "/define:DEBUG". Also, you can define your own symbols in order to accommodate your needs.
Define as many build configurations and symbols you need but don't abuse this feature!