Sets the data rate in bits per second (baud) for serial data transmission. For USB baudrate is fixed on 115200 bps, on UART it is possible to select baudrate. Valid values are: 9600, 14400, 19200, 38400, 57600, 115200, 230400 bps For lower baudrates use Software Serial
Serial.begin(baudrate) // for UART
Serial.begin()         // for USB
baudrate baudrate to be used. If omitted, default 115200 bps is used (the only available on USB interface) nothing
void setup() {
    Serial.begin();      // opens serial port on USB, sets data rate to 115200 bps
    Serial1.begin(9600); // opens serial port on UART1, sets data rate to 9600 bps