In C++, a function is a named block of code that performs a specific task. Functions are used to organize code into reusable blocks, which makes programs easier to write, read, and maintain.

A function in C++ typically has a return type, a name, a list of parameters (optional), and a body:

return_type function_name(parameters) {
    // function body
    // ...
    return result;
}

Here's an example of a simple function that takes two int parameters and returns their sum:

int sum(int a, int b) {
    return a + b;
}

In this example, the function is named sum, takes two int parameters (a and b), and returns their sum. The return_type of the function is int.

Functions can be called from other parts of the program to perform their specific task:

int main() {
    int a = 2, b = 3;
    int result = sum(a, b); // result is 5
    return 0;
}

In this example, the sum function is called with two int arguments (a and b), and the result of the function call is stored in the result variable.

Functions can also be defined to have no return type (void), which means they do not return a value:

void printMessage() {
    std::cout << "Hello, world!" << std::endl;
}

In this example, the printMessage function does not take any parameters and does not return a value. It simply prints a message to the console.

Functions can be declared (i.e., their signature can be specified) without providing an implementation. This is known as a function prototype, and it allows the function to be used before it is defined. Here's an example of a function prototype:

int sum(int a, int b);

This declares a function named sum that takes two int parameters and returns an int. The function can be called from other parts of the program, but the function body is not defined yet.