If you are using a program on the Mac which has multiple windows open, you can minimize all of the windows for that program at the same time. Instead of just clicking the minimize button on each window, hold down the Apple key and minimize just one of them. When you do this, it will minimize them all with a single click.

The same thing applies when you are opening a program. If there are multiple windows minimized on the dock for a single program, instead of clicking on each one to open it, you can hold down the Apple key and click on one of them and all of the windows for that program will open.