Final answer:
The Serial.begin() function is used to start serial communication in an Arduino sketch, where it's necessary to set the correct baud rate to match the communicating devices.
Step-by-step explanation:
The function used in the Arduino to start the serial communication using the COM port is Serial.begin(). This function initializes the serial port and sets the baud rate for serial data transmission. The baud rate is the number of symbols per second that are transmitted. For instance, if you want to set up a communication at a baud rate of 9600, you would use Serial.begin(9600). It's important to match the baud rate on both the Arduino and the device it's communicating with for the data to be read correctly.
Other functions related to serial communication in Arduino include Serial.available(), which checks for incoming data, and Serial.write(), which is used to send data to the serial port, but these do not initiate the serial communication.
The setup() function, on the other hand, is used to initialize variables, pin modes, and other libraries needed in the sketch, but it does not start serial communication on its own.