what is the main idea

You are referring to the main themes in the story. There are a few main ideas that you can consider.

Home, wandering, and fidelity

The title of The Odyssey has given us a word to describe a journey of epic proportions. Throughout his travels, Odysseus' central emotion is loneliness. We first encounter him as he pines away for home, alone on Kalypso's beach, and he is not above weeping when thinking of home at other points. He also endures great loss through the deaths of his brothers-in-arms from the Trojan War and his shipmates afterward. Loneliness pervades the emotions of other characters; Penelope is nearly in constant tears over her absent husband, Telemakhos has never known his legendary father, and Odysseus' mother explains that loneliness caused her death.

Yet tempering Odysseus' desire to return home is the temptation to enjoy the luxurious surroundings he sometimes finds himself in‹particularly when he is in the company of beautiful goddesses. He happily spends a year on Kirke's island as her lover and does not seem to complain too much about his eight years of imprisonment on Kalypso's island. In both cases, Odysseus expresses little remorse about being unfaithful to his wife‹although infidelity is what he fears Penelope may be succumbing to at home.

That Homer never reproaches Odysseus for his extracurricular romances but condemns the unfaithful women in the poem recalls Kalypso's angry statement about the double standard for immortals: male gods are allowed to take mortal lovers, while female goddesses are not. Likewise, men such as Odysseus have some freedom to "wander" sexually during their geographical wanderings‹so long as they are ultimately faithful to their home‹while Penelope and the other women in The Odyssey are chastised for their lack of chastity. Indeed, Odysseus does remain true to Penelope in his heart, and his desire to reunite with her drives his faithful journey. Fidelity is also central at the end of the poem, when Odysseus tests the loyalties of his servants and punishes those who have betrayed him. You can check this and more out at the link below:

http://www.gradesaver.com/the-odyssey/study-guide/themes