What is the summary of "Before the Beginning of Years" by Algernon Charles Swinburne?

T.S. Eliot, commenting on Swinburne's poetry in an essay, said this about the first four lines of this chorus:

"This is not merely 'music'; it is effective because it appears to be a tremendous statement, like statements made in our dreams; when we wake up we find that the 'glass that ran' would do better for time than for grief, and that the gift of tears would be as appropriately bestowed by grief as by time."

Swinburne is explaining through verse that the brief existence of man's earthly life is compounded of many kinds of suffering. A lifespan brings sorrow ("tears"), pleasure is tempered with pain, summer's flowers fade, and so on.
Yet, Swinburne says, mankind does have beautiful gifts from the gods as well, although they are brief.

"They gave him light in his ways, And love, and a space for delight, And beauty and length of days, And night, and sleep in the night."

In the end (literally), however, Swinburne's view of mankind's existence is nihilistic. Man, he says,

"Sows, and he shall not reap; His life is a watch or a vision Between a sleep and a sleep."

Swinburne, like Shakespeare's King Lear, views mankind in this poem as essentially without free will. He is created and shaped by the gods, his existence is filled with all manner of mortal suffering, and then he passes from the "sleep" of pre-birth to the eternal sleep of death.

"Before the beginning of years" is the first line of one of the choruses from the play "Atalanta in Calydon" by Algernon Charles Swinburne. The play itself is a tour de force, an attempt to create an Aeschylean drama in English, not by translating Aeschylus but by choosing a similar theme (an important Greek myth) and then reproducing a lush, grand, and complex style of English verse that follows many of the principles of classical Greek prosody.
Basically, Meleager's father offends the goddess Artemis. Artemis sends a wild boar to ravage his land. Atalanta is a virgin priestess of Artemis and a renowned huntress. Both Meleager and Atalanta are part of a competition to kill the boar. They fall in love while hunting. Meleager kills the boar and honors Atalanta by giving her the spoils of the hunt. Meleager's brothers attempt to attack Atalanta, and Meleager kills them. Meleager's mother kills him in revenge for his killing his brothers. 
This particular chorus, like many of those in Greek tragedy, is a meditation on human nature. It argues that humans are moved by passions which can lead them to both joy and despair, love and hatred. It emphasizes the dualities of our natures and experiences and how our capacity for momentary greatness, love, and joy is balanced by our mortality.