The red pins represent clickable structures on the figure and structures list. You can turn these off by clicking the pin button.
On many images, you can quickly test your knowledge on the location of structures with the Quiz mode. Click the Quiz button to start. You will be asked to identify the structure highlighted on the image from a list of four options. Your answer is automatically marked and your score accumulates as you answer more questions. Your mark is not recorded, is cleared when you leave the screen and is not sent to your teacher.