What are predefined macros?

RE: AM335x SYSBIOS Industrial SDK 01.00.00.06 User Guide, Section 12, Building Bootloader.

http://processors.wiki.ti.com/index.php/AM335x_SYSBIOS_Industrial_SDK_01.00.00.06_User_Guide#Building_Bootloader.C2.A0

5. Set predefined macros as follows. For IDK, include HW_IDK  and remove SPI macro. For ICE, include HW_ICE and SPI.

What are predefined macros and how are they set? I'm using ICE.

Thanks, Ben