Click the serial monitor button in the toolbar and select the same baud rate used in the call to begin(). You can use the Arduino environment's built-in serial monitor to communicate with an Arduino board. Thus, if you use these functions, you cannot also use pins 0 and 1 for digital input or output. It communicates on digital pins 0 (RX) and 1 (TX) as well as with the computer via USB. All Arduino boards have at least one serial port (also known as a UART or USART): Serial. Serial is used for communication between the Arduino board and a computer or other devices.
Don't connect these pins directly to an RS232 serial port they operate at +/- 12V and can damage your Arduino board.
Serial communication on pins TX/RX uses TTL logic levels (5V or 3.3V depending on the board). Reference Language | Libraries | Comparison | Changes