To work with an application, you need to give the focus to its window. When a window has focus, any actions such as mouse clicks, typing text, or keyboard shortcuts are directed to the application in that window. Only one window can have focus at a time. The window that has focus will appear on top of other windows. It might also have a different appearance from other windows, depending on the theme selected.
You can give the focus to a window in any one of the following ways:
If the window is visible, click any part of the window.
On the bottom panel, click the Window List button that represents the window.
On the top panel, click the Window List icon and then choose the window you want to switch to from the list.
Press Alt+Tab. A pop-up window is displayed with a list of icons representing each window. While still holding Alt, press Tab to move along the selection list and release the Alt key to select the window.