Question

why do we call these mirrors as specular surfaces and that too perfect ones.
answer in detail please.

Answer 1

A specular surface is one that allows reflection on a nearly-flat surface that follows the rules of reflection. These include rules like the incident angle being equal to the reflected angle, and the incident, reflected and the normal being coplanar. 
Plane mirrors are perfect specular surfaces because these are absolutely plane and show perfect compliance with the laws of reflection.