Calendar categories allow you to overlay different types of events on your calendar. 

To add categories, click here to find out how

Once you have categories, you can select them when adding an event to your calendar: -

If you don't want to select a category then that's fine, but the option is there and what it means is that on the main calendar view, you can then change the view to show only calendar appointments/meetings which relate to that category

simple but effective to have you manage multiple calendars