Workspaces are containers for grouping projects. If you are using Decision Maker for yourself then you probably just need a "General" Workspace that you put all of your work in.
If you are using Decision Maker for a business then this is a way of breaking down work into particular areas. One way could be making each department that uses this a different Workspace. So an Accounting Workspace, an HR Workspace, a Procurements Workspace...
Creating a Workspace
To create a new Workspace click on the plus button on the top right of the page.