"French rockets go up, which unfold a silk parachute to the air and drift slowly down"

Parachute shells, also know as star or illumination shells, were non-lethal artillery shells that released a magnesium flare connected to a parachute.  The flare would float down and illuminate the battlefield below. They would be used most often to light up No Man’s Land and expose enemy troops on patrol. 

Colored flares without parachutes were fired to transmit orders.  For instance, a red flare might be a signal to commence a bombardment while a blue flare meant cease fire.