It is an extremely lightweight, flexible personal shielding system. DC Coats have been explicitly shown and stated to deflect energy attacks, most likely charged particle attacks, as well as nuclear radiation, though it's unknown if it only deflects particles or can deflect gamma radiation as well, and implied and shown in various battles to cancel out shockwaves.
Though called DC Coats, they need not always take the appearance of a coat, though that is the most common model. It is possible to wear more than one of the coat model, as shown by Dry in Ep0 - Ch21, but the manner in which he takes off the under coat after giving the overcoat to Anne before using his generation and then "Red Light - Crimson Pillar" may mean that DC Coats can hinder Paranormal Power.They appear to have classes, possibly 1-10, with the large bulky dark coat given to Anne by Dry on Valtia before he fights against Fear being a Level 7, Heavy Defensive Coating coat. (Ep0 - Ch21)