The meaning of Carl Sandburg's poem "Iron" is that even though instruments of war may be beautiful and romantic, and may excite the young men serving in the military, they are indivisibly connected with death and destruction.
The first stanza, which is almost a shape poem in that it builds out like a gun jutting from the side of a ship, emphasizes the joy the "jackies" (common sailors) feel in serving the "war god" and his shining instruments:
Glory of tan faces, tousled hair, white teeth,
Laughing lithe jackies in white blouses,
Sitting on the guns singing war songs, war chanties.
The next verse, much shorter, turns to another impliment of iron, the shovel. The allusion it has to make is indirect, requiring the reader to think for a moment and so increasing its impact:
Broad, iron shovels,
Scooping out oblong vaults,
Loosening turf and leveling sod.
The key here is "oblong vaults" -- these must be graves, by implication for the "laughing lithe jackies in white blouses" after they have been killed in battle. Thus, the final line, "the shovel is brother to the gun" means that war, no matter how romantic its tools may seem, inevitably entails death.