The big big blank space is the place you will be drawing or executing your programs. The smaller, grey part of the interface is the place where you will be writing the commands that the turtle(I will be talking about what a turtle is in just a minute) will be completing immediately, at least in 99% of the cases.
If you need any help using the commands, you can press "Help" at the top left part of the interface or save your program using the Save button.
As you can see in the blank space (Drawing space) there is a triangle. This is called a turtle.This is how a turtle looks like:
One thing that will prove itself usefuk over this course is the fact that you can write some simple math into the command writing space (i.e 2+6) and the program will give you a solution. This is one thing I like to mention so you don't have to open a calculator every time you want to draw something a bit more complicated!S