Loki was known as a trickster, shapeshifter, and all around trouble maker.
Once for every drop of venom in his eyes
As his punishment for tricking Hodor into killing his brother Balder and ensuring that he stayed imprisioned by Hel, he was chained to the world tree with a snake set above him to drip venom in his eyes. Loki's wife collects the venom in a bowl, but when she has to empty the bowl, the snake's venom drips in his eyes.
End, it has begun, now I am free,
Your ending sails with me
My serpent son stirs up the sea;
The Ship of Nails breaks free.
At Ragnarok, Loki will be freed from his chains, and come from the east in a ship made from the nails of dead men to join in the final battle.
Basically, this song is about Loki.
Shakes the ground in agony the Lord of Lies
Once for every drop of venom in his eyes
End, it has begun, now I am free, Your ending sails with me My serpent son stirs up the sea; The Ship of Nails breaks free.