The screen is where you organize your gradebook into different tabs for each period, subject, or section, and set some general options for your gradebook.
The Course or Subject is the course title. If you teach multiple periods of the same subject, select the same course for each, like "Algebra"; do not create separate courses like "2nd per. Algebra", since that defeats some of the other features. Also make sure the courses are consistent throughout the year so it can calculate cumulative grades, e.g., "World Lit" in 1st quarter and "World Literature" in 2nd quarter are not the same course. For more options see Schedule Courses in Admin Mode.
The Tab name is for the tab-bar in your gradebook. Typically for elementary this is an abbreviation, like "Lang", or the homeroom teacher like "Jones" if you're a special subjects teacher. For secondary it's usually the period, like "5th". For college it's usually the section, like "101 MWF". But this is completely custom. Students don't usually see this.
Notes: Tab names must be different for each class/section, otherwise problems will occur. Make sure your tab names are short enough for all of them to fit in the tab-bar, or set the Tab-Bar to "Show classes as Menu" if you have too many to fit.
It's normal to leave old gradebooks online so students and parents can see past grades, but you can check "All other gradebooks Offline" to hide all other gradebooks. (This affects your gradebooks only, so other teachers can have their grades online even if you don't.)