Although this Paul makes this statement in Chapter 11 near the end of the novel, he has been building up the support for it throughout the novel. The soldiers enter the war with little to no training, and as the war goes on, the soldiers who enter the front are younger and younger. Germany is steadily losing power in the war effort, so the government takes any soldiers that it can get. As Paul becomes more experienced, he begins to notice and reflect on the management of the soldiers. He realizes that men are not human in the war--they are simply tools to advance the war. By this time, Paul has lost many friends on the front, and memory of them is lost just as quickly as they die.
In the trenches of the first world war, the slaughter of young men was unbelievably large. Young men with almost no training were sent to the trenches and ordered to charge at the enemy's machine-guns. They were gunned down again and again and again.
In the last 9 years of war in Iraq and Afghanistan more than 5000 American soldiers have died. But on ONE DAY at the battle of the Somme (In France), over 60,000 soldiers were killed (and this mad killing went on for years)
So your quote is suggesting that the men who are being sent to the front are all the same... doomed, lost and they will die for nothing in their millions. They are all the same; merely fodder for the ever-hungry guns.
Anthem for Doomed Youth
What passing-bells for these who die as cattle?
Only the monstous anger of the guns.
Only the stuttering rifles' rapid rattle
Can patter out their hasty orisons.
No mockeries now for them; no prayers nor bells,
Nor any voice of mourning save the choirs,--
The shrill, demented choirs of wailing shells;
And bugles calling for them from sad shires.
What candles may be held to speed them all?
Not in the hands of the boys, but in their eyes
Shall shine the holy glimmers of good-byes.
The pallor of girls' brows shall be their pall;
Their flowers the tenderness of patient minds,
And each slow dusk a drawing-down of blinds.
by Wilfred Owen (who was killed a few days before the end of the war)