Imagine you are pedalling your bicycle on a level road. You stop pedalling: no force is now acting to move you forward. What happens? You gradually slow down. How could you slow down more suddenly, in a shorter distance? By putting the brakes on. Because the brakes change your movement, making you slow down more suddenly, they must be exerting a force on the bicycle and you, as they grip and rub on the wheel-rims. This is the force called friction, which tends to slow down moving things by acting in the direction opposite to movement, that is backwards. Even without the brakes on, there are other friction forces acting on you and your bicycle, which also slow you down. One of these is friction in the wheels rubbing on the axles. Another is air resistance, which you can feel, pushing you backwards as you and the bicycle move forwards. When you apply these ideas to something around you, like a cart, you can see what could be generating friction: mainly the axles rubbing on the body as they rotate.