Alt-Tab refers to a keyboard shortcut in Microsoft Windows that allows users to quickly call up another Window. Alt-Tab increases the speed at which a user can switch between windows. This short cut has also proved useful in hiding websites or programs when users surf the Internet at work.
Techopedia explains Alt-Tab
Like other commonly used shortcuts, such as Control-Alt-Delete and Control-C, Alt-Tab has taken on a pop culture meaning as a quick way to close non-work screens employees may be viewing at the office. Another common technique is to keep a generic spreadsheet ready at all times and either Alt-Tab it to the top of the display or close all open applications except the spreadsheet. Because there are so many ways to avoid detection, many companies choose to install monitoring software rather than trying to catch employees with YouTube up on their screens. Of course, there are ways around monitoring software too.