Panning means moving (scrolling) around in a drawing. The quickest way to do so is using the middle mouse button and move the view similar like you would move a paper around: press the middle mouse button and hold it while moving the drawing around. If your mouse does not have a middle mouse button you can press the Control key (Mac OS X: Command key) and use the left mouse button instead.
Alternatively, you can activate this tool and then move the view with the left mouse button without pressing any keys.
Click the right mouse button when you are done.