Tabs can let you open more than one entry at a time and quickly switch between them. To add a new tab, use the File menu's "New Tab" menu item. The tab is selected automatically with the current entry still selected. Any other entry that you select in this tab does not affect the selection in the previous tab. If you select the previous tab in the tab bar above the entry metadata area, the selection changes to the entry for that tab. Use the "Select Previous Tab" and "Select Next Tab" items in the View menu to switch back and forth between tabs quickly.
To close a tab, move your mouse over the tab and a close box will appear (with an 'x' in it). Click on that button to close the tab. If you hold the Control key and click on a tab, a contextual menu with extra actions will appear. Amoung other things, this will let oyu open that tab in a new window. It will also let you pin the tab to the currently selected entry. If a tab is pinned to an entry, then selecting another entry will create a new tab for that entry and leave the previous tab on the previous entry.