In short; you select a Project to work on in the Project spaces section. All first level projects are placed in the Project spaces section. In a single project space you create a hierarchy of items of different types.
In Categories you select if you want to work only with certain types of items within projects.
In the Filter you set view conditions such as viewing all items or just those that are not finished. There you also set boundaries for start and finish of items you like to view.
In the Sort section you simply decide in which order your items and projects appear.