Why moving things stop
Put a pencil on a table and flick it with your hand so that it slides along the table. You already know what happens: it slows down, and after a short way it stops.
Now put the pencil on a smoother surface than the table and flick it exactly as hard. This time it still stops — but only after travelling a good deal further.
The question worth asking
Nothing pushed the pencil backwards. No one caught it. So why does a moving object slow down and stop on its own, and why does the surface it travels over change how far it gets?
Something must be acting against the motion. An object slides over a surface, the surface tries to prevent it moving, and to do that it exerts a force on the object. That force is what this chapter is about.
Friction
The force a surface exerts to resist the motion of an object travelling over it is called the frictional force. It acts along the surfaces that are touching, and it is always opposite to the motion.
The pencil, answered
That is why the smoother surface let the pencil run further: less friction acting against it. Hold on to that idea — the rest of the chapter is really just careful work on how big this force is and what decides it.