"And the secret of steel has always carried with it a mystery. You must learn its riddle, Conan. You must learn its discipline...”
Conan the Barbarian

"No one in this world can you trust. Not men, not women, not beasts. (he points to the sword he just completed) But this... this you can trust."
Conan's Father, Conan the Barbarian

Only later in life would Conan come to the realisation of it's mystery. With Thulsa Doom's "Steel isn't strong boy, flesh is stronger" does he begin to see the full circle of the riddle.

Thulsa continues: "What is steel compared to the hand that wields it? Look at the strength in your body, the desire in your heart, I gave you this! Such a waste. Contemplate this on the tree of woe. Crucify him!"

The Riddle of Steel compares a man's heart to a piece of raw unworked iron. The heart must be hammered by adversity and forged by suffering, purged and hardened by the fire of conflict. A man's heart can only be purified and shaped on the "anvil of despair and loss".

We see in the movie that the father's sword is broken, perhaps representing one of the last links that bind Conan's chain of true freedom. Conan's real father was killed by Thulsa, and later avenged by Thusla's death. Conan beheaded Thulsa (using his father’s broken sword to do it) who beheaded his mother and, thus conquering his own world and finally willing his own will.

The death of his clan, Valeria, Thusla, the broken sword:

It is only when a Cimmerian man's heart has become as steel could he be worthy of handling a sharpened blade in victory against his enemies. If that Cimmerian man can master the mysteries of steel, his sword would become his very soul. Out of suffering births strength, only deprivation and pain can turn a heart as strong as steel.

At least, that's how I see it.