The Odyssey is an epic poem by Greek poet Homer, written at the close of the 8th Century BC, and acts as a sequel to The Iliad, although it focuses mainly on Odysseus instead of the aftermath of the Trojan War.
The first six books of the Odyssey are both a summary of what has come before and an explanation of what has happened in the meantime. The opening reads:
TELL ME, O MUSE, of that ingenious hero who traveled far and wide after he had sacked the famous town of Troy. Many cities did he visit, and many were the nations with whose manners and customs he was acquainted; moreover he suffered much by sea while trying to save his own life....
(Homer, The Odyssey, eNotes eText)
Odysseus is still lost at sea, and his wife Penelope is dealing with suitors who want to marry her and take the throne. Her son, Telemachus, is visited by Athena and becomes hopeful that Odysseus has survived; the bulk of the first six books details his efforts to gain aid for the search, his visits to Nestor and Menelaus to seek information, and finally switches to Odysseus, trapped for ten years on the island of Calypso.
Essentially, since the remainder of the book is a flashback to the many travels and obstacles he has faced, the first six books act as both a prologue and a summation -- "Previously, on The Odyssey."