All files named "main.conf" from all packages are appended together (in unspecified order), result placed on output directory of main package into main.conf.h file (always, even if it is empty). Path to this directory is placed to the include set. Ide/umk supporting this feature defines MAIN_CONF macro.
The purpose of this is to make possible a configuration of common packages (like Draw or CtrlCore) by adding other packages. For example, we can have package that provides custom font subsystem, this package would put "#define CUSTOM_FONTSYS" in its main.conf, which in turn would exclude generic font handling in Draw.