Horizontal scrolling is the ability of a program to allow a user to scroll horizontally using the window scroll bar. The horizontal scroll bar is not used often and usually not visible or not accessible because it is not needed. Below is an image of a Microsoft Windows window with a vertical and horizontal scroll bar.
In the above example, the horizontal scroll bar is visible, but is not accessible because word wrap is enabled and prevents any text from being entered beyond the visible window. If word wrap was not enabled and text was long enough to exceed one line the horizontal scroll bar would allow you to scroll left to right.