In the first lessons, you've had the Maker Board running the commands as fast as it can. In the rest of these programs, you will control the speed and order in which the commands execute. Think carefully about how delays fit in with your code, because two commands next to each other will execute so quickly to seem simultaneous. Without a delay, something can happen so quickly that you won't see anything change, which makes those issues even harder to troubleshoot.
Try moving the delay line to somewhere else in the code, like between the lcd.clear() line and the lcd.print() line. The entire program is still running, but you do not pause after printing Hello World, so it is immediately cleared before you see it. See how the placement of a delay makes a big difference in a program?
Restore the code to it's original state. Now type a new delay(100); between the lcd.clear() line and lcd.print() line. How does that affect the program?