The poem in the beginning of the book forshadows the entire plot of the novel:
On the day of the dead, when the year too dies
Must the youngest open the oldest hills
Through the door of the birds, where the breeze breaks.
There fire shall fly from the raven boy,
And the silver eyes that see the wind,
And the light shall have have the harp of gold
By the pleasant lake the Sleepers lie
On Cadfan's Way where the kestrels call;
Though grim from the Grey King's shadows fall,
Yet singing the golden harp shall guide
To break their sleep and bid them ride
When the light from the lost land shall return
Six Sleepers shall ride, six Signs shall burn
And where the midsummer tree grows tall
By Pendragon's sword the dark shall fall.
Will Stanton, who is the youngest of the Old Ones, returns from the "lost land" to his ancestral home of Wales. There he meets Bran, the "raven boy", and his dog Cafall with the silver eyes. The dog "sees the wind" and prevents a serious accident. Will is predestined to become part of the epic fight against the powers of evil -- both his friend Bran and the dog Cafall help him significantly in this fight. Their goal is the harp of gold, which Will finds.
The Grey King is the name of the fog that hangs around a mountain near where Will is staying with his relatives, but it is also the name of the supernatural being which Will has to fight. The Sleepers are servants of the Light, which Will must awaken to help in the fight.
Will and Bran do not understand the poem when the book starts, but as events unfold the plan -- and their place in it -- becomes clearer.