What causes an object to slide down a hill? Gravity, as we all know. How fast does it slide down the hill? That is determined by two things: the slope of the hill and how hard the object "pushes" against the hill.
The force of gravity is always directed straight down, toward the center of the Earth. Near the surface of the Earth the magnitude of the force of gravity is about . The slope of the hill determines how much of that force is "effective" in inducing the object to move: The steeper the hill (equivalently, the bigger the slope) the greater the component of the gravitational force which is causing the object to slide. Similarly, the slope of the hill determines how hard the object pushes down onto the hill: The gentler the hill (equivalently, the smaller the slope) the harder the object pushes on the ground, and so the harder it is to move the object.
These two parts are normally called the tangential and normal components of gravity, where "tangential" means "parallel to the hill" and "normal" means "perpendicular to the hill". The demonstration below illustrates how these components act on an object (in this example, a box) sliding down a hill.
Use the slider to change the slope of the hill. Observe that when the slope is small, the tangential component of gravity is small, while the normal component is large. Together, these mean that the object is less likely to slide, or at least will only slide slowly. Conversely, when the slope is large, the object will slide easily, as the force holding it back - the normal component of gravity - is small, while the force causing it to slide—the tangential component—is large.