Some of our courses (the longer ones) have the navigation sidebar on 'open' when they start, to give you an idea of how the course is structured etc.
Other courses (the smaller ones) do not have the navigation sidebar open as a default.
Sometimes, when the sidebar is open, it can block the view of some course material (especially pop up boxes).
You are free to close and open the sidebar anytime you wish. At the top of every lesson, there is a small button which will open and close that sidebar for you. Do close the sidebar anytime it gets in the way! You can always open it again, if you wish.