The main function serves as the starting point for program execution. It controls program flow by calling other functions. A program typically ends at the end of main. All C programs must have a main function which takes no arguments and returns an int. Main contains the core logic that runs the program. Preprocessor directives like #include add functionality by including header files. Macros defined with #define are text replacements that occur before compilation. Conditional compilation with #ifdef/#ifndef includes or excludes blocks of code based on symbol definitions.