Those 5 pads with the big holes are labeled on this side of the board. You will use these to attach various external devices, typically using alligator clips.
The first 3 are labeled 0, 1, and 2. Each one of these is like the other, they can either be an input or an output. And you get 3 of them to use.
These next two are a little different. They are labeled 3V and GND and are related to power. The 3V stands for "three volts", which refers to the voltage level of the power source. The GND stands for "ground", because everybody needs something to stand on, right? Three volts can't just float in space!