As a GM, when you create or open a map, it's shown as a tab in the tabbed area that takes up the bulk of the EpicTable window. 

GMs and players alike can drag the tab headers to reorder them or even drag them out of the tabbed area to arrange them differently or put them on a different screen. This is all personal to each participant. Reordering or rearranging the tabs has no affect on the other participants.

GMs can do other things with tabs, such as closing them. Closing a tab doesn't delete it. It just gets it out of your way. It's still in your game--you might have ten maps but only have one or two open at a time. If you want to delete a map or tabletop, you need to do so from the Maps and Tabletops tab on the ribbon menu at the top of the window.

The fact that you have a map (or tabletop) open in a tab doesn't mean the players can see it. If you create or open a map (or tabletop) as private, only you'll see it until you share it. You can share or unshare a map by right-clicking the tab header and choosing Shared or Private. 

As you an see, there are a number of other options available on the right-click menu. Most of these deal with managing your tabs. The Focus item allows the GM to request that the players sync to his view (map or tabletop tab and the zoom and position on that tab).