This window shows all invalid Database objects. Using this window you can recompile these objects. An empty window means no invalid objects exist.
Operations such as upgrades, patches and DDL changes can invalidate database objects. If these changes don't cause compilation failures the objects will be revalidated by automatic recompilation when accessed, but this may take some time to complete. For this reason it makes sense to recompile invalid objects in advance. It also allows you to identify if any changes have broken your code base.
Right-Click in the form and select one of the options Compile Selected or Compile All