Now, lets see just how fast our Arduino can communicate. Change the baud rate in the sketch from 9600 to 115200 and upload it to the board again. Then after opening the Serial Monitor select 115200 from the drop-down list.
Also, try mismatching the baud rate - use Serial.begin(9600) and select 57600 in the dropdown menu, for example. See what it looks like? All garbled? That's a mismatch. If you're ever getting strange data in the Serial monitor, triple check your baud rates and make sure they match!
The Serial Monitor is also a great way of debugging a sketch that is misbehaving. Sometimes, when a sketch is not behaving how you think it should behave, placing Serial.println() statements in key places will help you see what is going on.