The song is about a man who didn't want to be replaced by a machine, something many people since then have been able to relate to.

His job can't have been much fun, deep inside a tunnel, breathing kerosene lamp fumes, endlessly hammering on a steel rod pressed against solid rock, with the harsh metallic sound of the hammer blows greatly amplified and echoed by the rock-hard surfaces and small size of the tunnel. But it was the only thing that gave meaning to his life, and the only way he could earn a decent living.

He naively thought that if he showed that he was faster than the machine he could keep his job. We know that the steam drill did in fact replace John Henry, but rather than deal with the idea that management favors machines over people for other reasons besides efficiency, we explain that by saying that he died during the contest.

But it's absurd to suggest that someone who spent all day every day of his career hammering a steel rod into rock, creating miles of tunnels by the combination of the holes he drilled and the dynamite sticks placed in those holes, could be killed by driving a rod a mere 16 feet, no matter how quickly he did it.

The real danger in this operation is to the shaker -- the guy who has to hold the rod while someone swings a nine pound hammer at it, and has to watch the hammer head coming at him and move the end of the rod to the exact point where it will meet the arc of the hammer swing.

Think of how difficult that would be, especially in a dimly lit tunnel. And think of what would happen to you the first time you failed to put the end of the rod in exactly the right place. And think of the added danger when the operation is speeded up recklessly for a race.

Hence,
John Henry said to his shaker, "Shaker, you had better pray.
If I miss your six feet of steel, tomorrow will be you buryin' day."

And the shaker said right back to John Henry, "A man ain't nothin' but a man.
But before I would let any steam drill beat me down, I would die with my steel rod in my hand."

The idea that John Henry would work so hard for that short time that he might die with his hammer in his hand makes no sense, but it allows us to kill him off in the song, like an inconvenient movie character, so that we don't have to explain why the steam drill, which only made 9 (feet), replaced him anyway.