What means press and release the secondary mouse button?

On a Mac the Secondary Mouse Button is what is often called a Right Click - The primary button being the Left Click. When you click the Secondary/Right hand button you will see a menu of options appear from the mouse pointer. You can move your mouse down the list of options and click on an appropriate item. The contents of the menu will vary depending upon which application you are using when you click - which is why the menu is known as a Context menu. In the Mouse section of System Preferences you can configure the Secondary button to perform some task, other than activating the menu, like opening Spotlight to search for something.