Theory:

Imagine you are riding a bicycle; what will you do to stop or slow down the bicycle?
  
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Cyclist applies a brake to slow down the bicycle
 
We will apply the brake to stop or slow down the moving bicycle or vehicle. Have you ever thought about why a vehicle stops or slows down when we apply brakes? Not only vehicles, but any object that moves over the surface of another object also slows down when no external force is applied to it. In the case of a bicycle, when brakes are applied, the brake shoes hold the rim tightly, thereby arrests the motion of the vehicle.
 
To understand this concept, gently move or roll down a ball over a flat surface. Initially, the ball will move at a certain speed; later, the speed reduces and finally, it stops. This occurs due to friction.
 
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Ball rolls over a flat surface
  
Friction:
Friction opposes the relative motion between the surfaces, which are actually in contact with each other.
Now let us consider an activity to understand the term 'friction',
Gently push a book which is kept on a flat surface like a table. The book will stop moving after reaching a certain distance. It denotes a force that acts opposite to applied force, and this force is called friction. When we push the ball along the right direction, the friction will act along the left and vice-versa.
 
Why does a person fall when he steps on banana peels?
 
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A person steps on banana peel
 
When we step on a banana peel, we loose our balance because the banana peel's inner side is very smooth and slippery that reduces the friction between the sole of our shoe and the surface of the road.
Reference:
https://www.maxpixel.net/Rider-Bicycle-Cyclist-Cycle-Sport-Bike-Biking-1539793