The general form of this directive is:
#ifdef identifier
where identifier is your choice. If the specified 'identifier' has been defined by a #define directive, even with a null substitution, then any lines which follow this directive will be given to the compiler up to the properly nested #else directive or #endif directive.
If the identifier has not been defined then those same lines will be skipped and not compiled.
This directive will probably be dropped from the preprocessor in the future and be replaced by the #if directive combined with the 'defined()' operator.