I am a new Admin with Monday.com, so please be patient, I’m not as smart as I look.
We can create Main Boards and Private Boards. But Workspaces are either Open or Closed. Why aren’t Boards “Open or Closed”? And in the Content Directory boards are Private or Public. Why is it called “Public”, I thought they were “Main”?
I’m easily confused . . . .
Hey Gerald!
No worries at all - it can absolutely be confusing, especially when you’re getting yourself acclimated to monday! 
Workspaces being open means any team member can choose to join any open workspace on their own. On the other hand, closed workspaces are private and accessible only to the person who created them. Team members can join by invite or by requesting to join.
This is very similar to how boards work, however, there are 3 types of boards versus 2 types of workspaces so that is one reason behind the different names! Anyone can join a main board, like an open workspace.
However, private boards differ from closed workspaces in the way that the private boards are not visible or even searchable if you’re not invited to them, whereas a member can see a closed workspace that they are not apart of and request access.
It gets confusing when you add in the Content Directory but that is a feature only available to Enterprise account admins - like yourself!
In the content directory, you can see an overview of all the Workspaces, Boards, Dashboards, and Workdocs located in the account.
For each of these features, you can see its owners, subscribers, creation date, last updated date, and whether it’s publicly available to the rest of the account members or not. Private boards are not publicly available and can only be viewed by the subscribers of that specific private board. Admins of an Enterprise plan will be able to view high-level details of a private board such as its name, subscribers, and creation date via the content directory.
So even though boards fall into one of three categories (private, shareable, or main), the Content Directory helps admins see all the details of all boards - including private boards which would be otherwise unavailable to you unless you were explicitly invited to them.
Does this help clarify? My apologies if I cause any further confusion! 
Let me know what you think!
Help me understand . . .
what’s the point in being a member of a workspace if anyone with a license can see all main boards anyway?
Seems to me that you should only be able to see main boards in workspaces you are a member of.
As Monday is a collaboration tool, it probably makes sense that all members of the account have access to all open workspaces. Within the workspace you can still work with Private/Shareable boards.
On Enterprise version you have the possibility to create a so-called Closed workspace. The owner needs to invite members to that workspace. It can be found by the members of the account, but not accessed. The real downside of a Closed workspace is that you can not invite guests (from outside of your account) to that workspace.