That ball is what makes the mouse work. As you move the mouse to the left, the arrow on the screen moves left. Move the mouse right and the arrow moves right. Move the mouse away from you and the arrow goes up, etc. A light touch on the mouse is easier on both you and the mouse.
Every mouse has one or more buttons on top.
On a standard PC mouse there will be two buttons. You will only use the left hand button. The arrow that is moved by the mouse, and the button you click, allow you to make the Browser do many things. These actions will be demonstrated and used repeatedly during the workshop. The line of words across the top of the Web Browser program.