It is the infra red focus adjustment. When you focus with visible light but take image with IR, you sometimes need to adjust the focus. Some lenses, like some L lenses, don't require this. The red mark has nothing to do with what you are trying to do.
If you look at the DoF equations, you can see that when the focus distance is near the hyperfocal distance, the DoF is about evenly split fron to back. As you move in this changes gradually to about 2/3 in front and 1/3 behind the focus distance. (I did this from memory. It may be reversed, but you can see that under the correct assumptions, either 1/2 or 2/3 can be correct. ANoth good reason to understand the assumptions being made by others on your behalf to try to make your life easier. But sometimes they make just conuse the actual situation.)