The title comes from the last two lines, and the lines are Latin and translate roughly to "It is sweet and right to die for your country."
My friend, you would not tell with such high zest(13)
To children ardent(14) for some desperate glory,
The old Lie; Dulce et Decorum est
Pro patria mori.(15)
Basically, this is an anti-war poem. These last lines refer to the fact that people often describe war as a wonderful thing, and as the Latin suggests, glorify dying for one’s country. The poem does not agree with this, calling it a “Lie” with a capital L. Children deserve to know the truth: that war is terrible. Instead of glorifying war, this poem describes it in its horrid detail and encourages people to appreciate and understand the difficulty that war causes for a person and a family. Too many children run off to join up in the war, when they see it as romanitc and don't really know what they are in for.