I know that this is to print something:
But I want to know what it really means?
Both sides of the serial connection (i.e. the Arduino and your computer) need to be set to use the same speed serial connection in order to get any sort of intelligible data. If there's a mismatch between what the two systems think the speed is then the data will be garbled.
9600 bits per second is the default for the Arduino, and is perfectly adequate for the majority of users, but you could change it to other speeds: