Member functions (also called methods) define what an object can do. They have direct access to all member variables without needing parameters. Inside void Car::drive(), you can read and modify speed directly.
You can declare functions inside the class or just declare them and define them outside: void drive(); inside the class, then void Car::drive() { speed += 10; } in the .cpp file. This keeps class definitions clean.
Const member functions promise not to modify the object: void display() const; This lets you call display() on const objects and communicates that the function is read-only.