This is to exercise your eye muscles and increase their flexibility. This is quite a simple exercise but a good one. Imagine a giant figure of eight in front of you about 10 feet in front of you. Now turn the 8 on its side.
Now trace the figure of eight with your eyes, slowly. Do it one way for a few minutes and then do it the other way for a few minutes. It may seem very alien at first but it is worth persevering with it.