Serial monitor is used to communicate with computers,other arduinos or any other devices. Arduino Uno have 2 serial pins, 0 and 1. 0 is for Rx (receiver) and 1 is for Tx (transmitter).
Connecting any other devices to these pins will interfere with communication of computer with arduino, which results in wrong upload of code to arduino. Hence it’s always advised to first upload code onto arduino boards and then connect with other devices.
Generally, the baud of the serial monitor will be set to 9600,hence we call serial.begin(9600) in the void setup loop. You have various number of bauds, that include 300 , 600 , 1200 , and so on till 115200 (300 multiplied by 2 gives 600, when multiplied by 2 gives 1200, and so on). Baud rate is simply the speed of communication.