This is a drop-down that allows you to switch between different companies you've been added to or are doing work for.
You can find the switcher in upper left side of both the Desktop and Viewer tabs. Clicking it in either location will open up a dropdown menu listing all of the companies you're a part of.
Changing companies in the Desktop switches you over to the new company with all of the Groups and Folders closed. Changing Companies in the Viewer opens up a previously cached folder.
If you're across multiple companies and are unable to see a Production or Folder that you've been added to, you may not be viewing the right Company.