As for structure, you have 3 stanzas following the rhyming pattern of AA/BB, meaning, lines A rhyme and lines B rhyme.  As for content, stanza one describes a happy, cheerful young lad.  Stanza two throws us the bad news:  war has turned the happy young lad into a miserable man, who chooses suicide over surviving one more day in the awful trenches of war.  The blunt presentation of that action, "He put a bullet through his brain", makes the reality of it seem even more harsh.  Stanza three speaks to "You snug-faced crowds with kindling eye/Who cheer when soldier lads march by", rebuking them, telling them to "Sneak home and pray you'll never know/The hell where youth and laughter go."

It's a rather straightforward, heart-wrenching poem that directly confronts the horrors of war.  The last stanza really drives it home, contrasting the cheering crowds to the brutal reality, the "hell" that war can be.