Question

why the entering of trachea is guarded by epiglottis

Answer 1

 The glottis is guarded by epiglottis because it prevents the entry of food in wind pipe. If the food enters wind pipe, then it chokes it, leading to the death of the person. 

Answer 2

The epiglottis is a flap that covers your larynx so that food/drink doesn't go through the respiratory system while eating.

hope it helps