The pen is not visible on the canvas, so, it helps to visualize the pen movement by writing a few commands and then running the script to see the result before continuing.
The drawing starts with the active position at the middle of the canvas and facing NORTH or upwards. The default pen mode is down, which means that it is ready to draw when you use movement commands.
In Turtle graphics, drawing is done using the current pen position as the frame of reference. For example, to draw a line, you do not specify the start and end positions of the line. Instead you specify the length of the line and it is drawn from your current pen position. This change in perspective allows the commands for creating graphics to be reduced and simplified, yet, still able to create complex visual results.
Drawing is performed by moving the pen. When the pen is in the down mode, each forward or backwards movement creates a line of the specified movement distance. By combining the movement commands, you can create lines of any size or shape.
Drawing is done using the current pen position as the frame of reference. When you say move forward 100 pixels, that creates a line 100 pixels in length, starting from the current pen position. This approach enables you to create lines without having the specify the start or end points because they are automatically calculated based on your current position and the length of the line.
To create more complex lines, curves and shapes, you change the direction of the pen. For a new script, the default pen position is the middle of the canvas and the default pen heading is NORTH or upwards. So, turning 90 degrees to the right will change the pen heading to the equivalent of EAST or facing to the right of the screen.
For a new script, the pen defaults to the down mode. That means each movement command will create a line. When you want to move the pen to a new location without making a line, you just change the pen to the up mode.