The turtle knows instructions like forward 100 - this "tells" the turtle to move forward 100 steps. Try it! And of course back 100 moves the turtle back 100 steps.
pd tells the turtle to put the pen down. Try forward 100 now. Check.
And you can tell turtle to turn - right 90 - means turn 90 degrees.
Exercise 1 draw a square. Check. Notice how my turtle ended up facing the same way as it was in the start.
Computers are very good at repeating things - they don't get bored! Notice how you said forward 100 right 90 four times.