A workspace is where your software architecture model and diagrams reside. The list of workspaces you have access to is available on your dashboard after signing in.
Click the "Show more..." button to expand the workspace summary. Depending on your access level to the workspace, your dashboard will provide a summary of the workspace, the API key/secret pair, the number of users who have access, etc.
As the owner of a workspace (the user who originally created it), you have the ability to make your workspaces public or private, regenerate the API key/secret pair, delete the workspace, and manage the list of users who have access to the workspace. Users with other levels of access (admin, read-write and read-only) will see different details and/or buttons as appropriate.