The mini toolbar appears whenever you right-click text in Word, Excel or PowerPoint.
The mini toolbar provides a quick-access version of the Font group on the Home tab, plus a few extra buttons from other groups. Hover your mouse over each button to find out what it does.
In Word and PowerPoint, the mini toolbar also is available when you select text; a faint, partly transparent version of it appears above the selected text. If you move the mouse over it, it becomes solid and usable. If you find it annoying that the mini toolbar appears for selected text (some people do), choose File-->Options and then deselect the Show Mini Toolbar on Selection check box.