This question is wide open. Explaining a poem could mean any number of things. It could mean explain structure, organization, rhythm, rhyme, meaning, symbols, etc. I'd like to focus a bit on the structure and how the poem moves from setting to setting within the poem's unique structure. Notice that stanzas 2 and 4 are indented. They visually call attention to themselves by standing out from stanzas 1, 3, and 5. Crane isn't doing this on a whim. When a poet changes something in a poem, savvy readers should take special note and look a little deeper. The odd numbered stanzas are all being spoken to a specific person. The person is the loved one of a solider that has died in battle.
Because your lover threw wild hands toward the sky
The even numbered stanzas then change the topic and setting. Readers are taken to a battlefield that is filled with thousands of soldiers and a cacophony of violent, booming noises.
Hoarse, booming drums of the regiment,
Little souls who thirst for fight,
These men were born to drill and die.
The unexplained glory flies above them,
Great is the battle-god, great, and his kingdom—
A field where a thousand corpses lie.
These indented stanzas are much less intimate than the other stanzas. The poem is filled with verbal irony because war is absolutely not kind, and the poem highlights a few of the graphic details from battle that prove the point that nothing about war is kind. I'm not sure what voice Crane wanted his readers to read this poem with, but I always tend to read it out loud to students with a really sarcastic and snarky tone in a lot of places.