Working sets is a general Eclipse concept for grouping elements of different kinds, for example projects. Many views and commands enable you to filter the information shown and operated on by specifying a working set. Only elements that belong to the specified working set are shown by the view or operated on by the command. For example, use a working set for filtering the Project Explorer, to only show a subset of all projects in the workspace.
A good article that explains working sets, and how they are supported by different Eclipse views, can be found here.
By default, working sets are static, meaning that newly created elements are not automatically added to existing working sets. A view that is configured to filter on a particular working set therefore only shows the newly created element if it is a child of another element that belongs to the working set. If you want to have working sets that are dynamic, you might consider installing this plugin.