A function is a block of code that you can run from different parts of your program. If you have code you need to use in multiple places, you put it in a function. You can then call the function whenever you need it. When a function is called, the program runs the function's code and then goes back to where it left off.

Example:

#include 

// Function declaration
void greet();

int main() {
    printf("Program started.\n");
    
    // Calling the function
    greet();
    
    printf("Program ended.\n");
    return 0;
}

// Function definition
void greet() {
    printf("Hello, welcome to the program!\n");
}

In this example, the greet function is defined separately. When greet() is called in the main function, it prints a welcome message. After the greet function finishes executing, the program continues with the next statement in main().