Everything is clearly explained in the documentation: CSS scroll snap. The example Scroll snap in action is pretty clear: you can look at some large scene with objects through a viewport.

If the objects' layout is regular, you may prefer not smooth scrolling, but scrolling snapping at certain object features, for example, centering each object in the viewport, see the objects aligned by one of the sides of their bounding rectangles, and so on.

To see the difference in behavior, check or uncheck the checkbox disable snapping, use both the arrows and the sliders of the scrollbars.