It is a good design practice to assign meaningful names to I/O pins instead of just referring to them by number. If, for some reason (such as changing microcontroller boards), you need to change I/O pin assignments, you will only need to make the change in one place.
// Name an I/O pin with a preprocessing macro
#define SENSOR1 3 // Some kind of digital sensor input
// Name an I/O pin with a constant
const uint8_t ACTUATOR1 = 4; // Some kind of digital actuator output
void setup()
{
pinMode(SENSOR1, INPUT);
pinMode(ACTUATOR1, OUTPUT);
pinMode(2, OUTPUT); // Unnamed I/O pins are discouraged!
}
void loop()
{
}