There are two ways you can configure the scroll direction on a computer: normal or inverted (what Apple calls “natural”). If you, like most, use a mouse with a wheel, the normal scrolling scheme is probably what you use. Under it, moving the mouse wheel down (i.e., so that a point on the top of the wheel moves down/closer to you) causes the content to move up and the viewport (the window into the content) to move down. Similarly on a multitouch trackpad, under normal scrolling, as your two fingers move down, the content moves up and the viewport down.
The other option—natural scrolling—flips this, so as your fingers move down on the trackpad, the content moves down and viewport moves up, and similarly for rotating the mouse wheel. When using a mouse, this feels to most people obviously backwards. But this setting doesn’t exist without reason. It’s transposing the paradigm of touchscreens on to the trackpad, where your finger remains pinned to the point in the content where you first touched. You move the content directly, rather than moving the viewport.
This generally isn’t a big deal; most people just find the mode they prefer, change the setting, and then never touch it again. But what if you prefer both?