You've just finished the Buttons less and you're still not getting what the resistor thing was all about. Okay, this is for you.
Start by thinking of A0 as being able to read the voltage difference between GND and whatever is plugged into it. If you plug a jumper between GND and A0, it's going to read 0v. The reason is simple, the potential difference between GND (0v) and GND (0v) is 0v.
If you have a multimeter, imagine that A0 is the red probe, and GND is the black probe.
When you call digitalRead(...), the ATMega328p is checking if the voltage is over 3v. If it is, then you get a positive result from the call.