Because of friction u are able to walk or run.When a force is applied to an object, the resistive force of friction acts in the opposite direction, parallel to the surfaces. for example in soccer, when a ball is brand new, your foot sticks to it, increasing friction and making you slip. 
When the grass is too short, there isn't enough friction to stop it and it moves too fast.while running or walking, it is the friction which control your movement and help you to walk in a better way or u are expected to just side off without friction.
Same as above as you can see how it is beautifuly explainee.
Thank you