Serial connection is established between your Arduino board and another device using begin. Serial communication is most commonly used between your Arduino and your computer through a USB wire (or, should I say, a Universal Serial Bus cable). The other end of the cable is called the serial port. There can be only one serial port on a PC. If you have more than one Arduino connected to it, more than one set of instructions will be executed. The begin function opens a connection with another device based on the specified parameters. It returns true if the connection is successfully made, false otherwise. By default, the first serial port will be used by the Arduino board. You can check this by printing out the value of serialAvailablePorts. If no ports are available, an error message will be printed out. You can fix this by adding some code that makes sure a serial port is available before making a connection. Once a connection has been made, data can be sent back and forth between Arduinos and PCs via their corresponding serial ports. For example, you could send text messages from your computer to your Arduino over Bluetooth or wireless Ethernet connections. Or you could read sensor data from a remote temperature sensor installed outside and write it to the arduino's memory card through a USB port.