Question

why a cycle can slowed down by dragging our feet along the ground

Answer 1

Friction! The direction of the moving cycle is opposite of the friction the biker applies. So,the cycle slows down and stops. Also,for instance- I roll a pencil on a table and my friend stops it with her finger.here,she applies an opposite force to the direction of the rolling pencil to stop it. In your case,dragging of feet on ground gives enough friction to halt the cycle. Cheers!