Apple Macintosh PowerBook 145 - Manual (Page 49)

Click
You click an object on the screen by pressing and quickly releasing
a trackball button while the pointer is over that object.
Practice clicking different objects on the Macintosh desktop, such
as the startup hard disk icon near the upper-right corner and the
Trash icon near the lower-right corner.
When you click an icon, it becomes highlighted (the icon is
darkened). A highlighted icon is said to be selected. A selected
icon is the object of whatever action you choose next.
Chapter 2: Working on the Desktop 33