Variable Declaration:

• char userInput = 0;
  This variable is used to store the incoming data from the serial communication. It is initialized to 0 and will later hold the character received from the serial input.

Setup Function:

• Serial.begin(9600);
  Initializes serial communication at a baud rate of 9600. This allows the Arduino to communicate with a computer or other devices via the serial port.

• pinMode(9, OUTPUT);
  Sets pin 9 as an output pin, meaning it will send voltage out to control an external component (e.g., an LED or motor).

• pinMode(8, OUTPUT);
  Sets pin 8 as an output pin, meaning it will send voltage out to control another external component.

Loop Function:

• while (Serial.available() == 0);
  This line waits for data to be available in the serial buffer. The program pauses here until the user sends data via the serial communication.

• userInput = Serial.read();
  Reads the incoming data from the serial buffer and stores it in the userInput variable.

• Conditional Statements:

  • if (userInput == '1') { ... }
    If the received character is '1', pin 8 is set to HIGH (turned on), and pin 9 is set to LOW (turned off). This could be used to control two components, such as turning on one LED while turning off another.

  • else if (userInput == '2') { ... }
    If the received character is '2', both pin 8 and pin 9 are set to LOW (turned off). This could be used to turn off both components.

• Serial.println(userInput);
  Prints the received character (userInput) back to the serial monitor. This is useful for debugging or confirming the input.

• userInput = '0';
  Resets the userInput variable to '0' to prepare for the next iteration of the loop.

The Circuit didn't work at first but it turns out  the problem with the circuit was due to incorrect wiring between the Bluetooth module and the Arduino Uno. 

 the TXD (Transmit Data) pin of the Bluetooth module must be connected to the RX (Receive) pin of the Arduino Uno, while the RXD (Receive Data) pin of the Bluetooth module should be connected to the TX (Transmit) pin of the Arduino Uno.