The best way to understand how the turtle moves is to imagine that you are the turtle.
If you hear the command forward 10, you, most probably, will make 10 steps forward.
That's the same what the turtle will do. The only difference is that its steps are much smaller than yours, and the name of the action 'forward' in actually shortened to fd.
In a similar way going backwards is done with the action bk. The following instructions will ask the turtle to go 10 steps forwardand then to go backward 5 steps.