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, so nothing covers any part of it. It may also have a different appearance from other windows, depending on your choice of theme.
You can give the focus to a window in any of the following ways:
With the mouse, click on any part of the window, if the window is visible.
On the bottom panel, click on the window list button that represents the window in the Window List.
On the top panel, click the window list icon and choose the window you want to switch to from the list. The window list icon is at the extreme right of the panel, and its icon matches that of the current window's Window Menu button.
If the window you choose is on a different workspace, you will be switched to that workspace. For more on workspaces, see Section 2.4 ― Workspaces.
With the keyboard, hold the [Alt] key and press the [Tab] key. A pop-up window appears with a list of icons representing each window. While still holding [Alt], press [Tab] to move the selection along the list: a black rectangle frames the selected icon and the position of the window it corresponds to is highlighted with a black border. When the window you want to see is selected, release the [Alt] key. Using [Shift+Tab] instead of just [Tab] cycles through the icons in reverse order.
You can customize the shortcut used to perform this action with the Keyboard Shortcuts preference tool.