Tabs allow you to work on more than one thing at once. A tab shows a single view. For example, you could have both the table view and the form view open at the same time in different tabs. To create a new tab, click the plus button in the tab bar. To close a tab, click the 'x' button in the tab you want to close. Tabs can be reordered by dragging them to a different location. However, tabs cannot be moved out of the window.
Shown at the right of the window, the overflow menu will appear when there are too many tabs to fit in the tab bar. Any tabs not shown will be moved to the overflow menu where they can be selected. To completely hide/show the tab bar, use the View > Tab Bar menu item.