In Autopsy, a "case" is a container concept for a set of images. The set of images could be from multiple drives in a single computer or from multiple computers. When you make a case, it will create a directory to hold all of the information. The directory will contain a configuration file, some databases, and some other information.
Refer to the Creating a Case page for more details.
There are three ways to open a case:
After that, a File Chooser dialog will show up. Then select a ".aut" file that you previously created. It will be in the case folder.
After you create a case, you can add an image to the case.
If you want to view case details or edit some case information, use the Case Properties window.