This week's book giveaway is in the OCAJP forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide 1Z0-808 and have Jeanne Boyarsky & Scott Selikoff on-line! See this thread for details.
In a GUI, normally one of the GUI elements (text boxes, buttons, etc.) has the focus, which means that if you type, the text appears in the text box that has the focus, or when you press space, the button that has the focus is clicked.
When the focus goes from one GUI element to another, for example because you press the Tab key, then the element that previously had the focus loses it, and it goes to the next GUI element.