Defines
Defines are processed during the compilation process. You can read about them in the pawn language guide on page 92.
Bitwise Bugsy talked a bit about this (it's amxx but applicable). As this is not a sourcemod exclusive thing (neither are the defines) i recommend searching for "bitwise operators" or "bitwise operators site:forums.alliedmods.net". You can also read a bit about it in the pawn language guide starting on page 104.