The copri-ferro is the amount or depth of the concrete that covers a rebar (the ferro). In English this is just called concrete cover.

http://www.concreteconstruction.net/how-to/construction/plac...
Cover
One important reason for placing the reinforcing steel properly is to achieve the right amount of concrete cover—the amount of concrete between the reinforcing steel and the surface of the concrete member. Cover is the single most important factor in protecting reinforcing steel from corrosion.

A look on a Google image search shows the meaning in Italian
https://www.google.com/search?hl=en&q=copri-ferro&tbm=isch#i...


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Note added at 13 hrs (2018-04-06 10:38:46 GMT)
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In Italian the "copertura" is often synonymous with roof something to cover above, so here I would take the copri-ferro as concrete cover above (copri-ferro) and on the sides. The reinforcement bars are not painted so if they are exposed in anyway they will rust and the concrete will lose its strength. Obviously shelter above is most important, but water and the atmosphere can get in from the side too.