When you have more than one workbook open, and you want to switch from the current workbook to another, it’s time-consuming to have to use the mouse to find and click on the one you want. This is especially true if the workbook names don’t all appear separately in the Windows taskbar.
A faster way is to hold down Ctrl and press Tab. This switches you to the last-used workbook.
If you keep Ctrl held down and press Tab again, you select the workbook used second-to-last. Keep Ctrl down and continue to press and release Tab to move through all open workbooks.