Click on the plus sign  on the left and choose between adding a Local workspace or Online workspace. Read more here about the difference between the 2 types of workspaces.
After you have clicked on Add a local workspace you need to insert the workspace's name  and location . You can also select the location of a previously created workspace which will be automatically loaded; alternatively, you can start from scratch and create a new workspace in a new empty folder.
When a new empty folder is selected, the workspace will be populate with a demo content called Introduction. These are just examples to show what you can do with Stacks, you can either edit them or deleted and create your own documents.