Hold down Option if you're on a Mac, and the Command key, too. Now click any icon in the Dock, and don't be alarmed.
Yes, all applications but but the one you clicked have disappeared. They're not gone or quit, though, just hidden in one convenient swoop; conveniently hidden, in fact, like all the Gmail labels you don't need right now:
›› Some of your Gmail labels are more important than others, and you want to hide shocking folders such as "Spam", too? Here's how to hide labels in Gmail and bring them back when you need them.