When you select text and point at it, the Mini toolbar will appear faded.
Some formatting commands are so useful that you want to have them available whatever you are doing.
Let's say you want to quickly format some text, but you're working on the Page Layout tab. You could click the Home tab to see the formatting options, but here's a faster way:
Select your text by dragging with your mouse, and then point at the selection.
The Mini toolbar will appear in a faded fashion. If you point to the Mini toolbar, it will become solid, and you can click a formatting option there.
The Mini toolbar is great for formatting options, but what if you want other types of commands to always be available? Use the Quick Access Toolbar. The next page will explain what it is.