The word flächendeckend can be found in any dictionary. It consists of two words.

flächendeckend       Fläche = surface or territory   deckend = participle of decken (= to cover) => covering

The direct translation would be: covering the (whole) surface or territory. The word flächendeckend can be used as adjective or as an adverb. And even if it is not an English word, that is to be found in a dictionary, one can easily imagine what flächendeckend.

flächendeckend       flächendeckend = covering a territory