Owen's aggressively anti-war poem uses the metaphor of a church service to frame the horrific scene of men dying, most likely in France, during World War I.
Instead of the sound of church bells summoning the faithful to church, the "passing bells" in the poem are the sounds of artillery fire, and these guns are killing the soldiers as if they are cattle. During a church service, the orisons are prayers, but to Owen, the orisons consist of the "rapid rattle" of rifle fire.
In the world of battle, normal aspects of a religious service--prayers, bells, choirs--become mockeries, the "demented choirs of wailing shells." Wilfred's use of the "bugles" has a much more sinister meaning than one would normally associate with a bugle call. In this case, the bugles come from the soldiers' funeral services held in the "sad shires" in which they lived, and the dead are so numerous, their are simply not enough candles to light the number of funeral services.
The last few lines complete this sorry scene: the farewells to the fallen soldiers are reflected not by the candles but by the eyes; the covering usually on the casket is replaced by the blankness on the faces of the soldier's girl friends and lovers; and the flowers are replaced by the tender thoughts of those left behind. The last line is especially sad because it describes the actions each night of those left behind--they draw the blinds down to shield the rest of the world from their sorrow at having lost a son, a brother, a father, a husband.