In the first stanza, the speaker talks about the poppy fields that are now transformed into battlefields and the loud noises of war that can be heard in the background—especially the sound of larks bravely singing in the sky and guns going off in the fields.

In Flanders fields the poppies blow
Between the crosses, row on row,
That mark our place; and in the sky
The larks, still bravely singing, fly
Scarce heard amid the guns below.

The rows of poppies actually symbolize the rows of dead soldiers, as well as death itself. The crosses are the graves of the dead soldiers, while the larks that are bravely flying and singing above the fields symbolize life. The singing of the larks unfortunately cannot be heard that well, because the guns going off in the fields below are too loud. Thus, the sounds that can be heard are the larks in the sky and the guns on land.

"In Flanders Fields" is a war poem and it's written in memory of all of the soldiers that lost their lives during the First World War. The phrase "Flanders Fields" is actually often used both in literature and history to refer to the battlefields of WWI in France and Belgium, where many soldiers were killed and buried.