what the metamaterial absolutely is?

A metamaterial (or meta material) is a material which gains its properties from its structure rather than directly from its composition. To distinguish metamaterials from other composite materials, the metamaterial label is usually used for a material which has unusual properties. The term was coined in 1999 by Rodger M. Walser of the University of Texas at Austin. He defined metamaterials as:[1]

Macroscopic composites having a manmade, three-dimensional, periodic cellular architecture designed to produce an optimized combination, not available in nature, of two or more responses to specific excitation.

Among electromagnetics researchers, the term is often used, quite narrowly, for materials which exhibit negative refraction.