#define is a type of preprocessor directive, meaning the compiler preprocesses it before compiling the code, thus taking up zero memory. The constant here is called the macro name (SENSOR_PIN or LED_PIN), and the value is called the macro value.

The reasons it's a better approach than simply using variables are:

1. They don't occupy any memory.
2. They improve code readability.
3. They can also be used with conditional directives (#ifdef, #ifndef, etc.) or functions to create code that behaves differently depending on certain conditions.

Hope this helps.

#define SENSOR_PIN A0
#define LED_PIN 13

void setup() {
  pinMode(SENSOR_PIN, INPUT);
  pinMode(LED_PIN, OUTPUT);
}

void loop() {
  int sensorValue = analogRead(SENSOR_PIN);
  if (sensorValue > 500) {
    digitalWrite(LED_PIN, HIGH);
  } else {
    digitalWrite(LED_PIN, LOW);
  }
}

Could someone explain to me what it does here and why to use it? Why not use this instead:

int SENSOR_PIN A0
int LED_PIN 13