When Odysseus is leaving to continue his voyage, Circe warns him about Scylla and Charybdis. Scylla is a six-headed monster and Charybdis is a whirlpool that swallows the sea and spits up hot water three times a day (lines 584-589). Circe says that it will be better to lose six crew members to Scylla, then all to Charybdis (lines 589-592).
Moving on in Rage
Once the ship gets close to Scylla and Charybdis, Odysseus tells his crew to row towards the mountain, but he doesn't tell them about the monster (620-621). Odysseus put on his cuirass (armor for the chest and back) and took two heavy spears, then went to the foredeck looking for the monster (625-631).
As the crew got closer to the rocks, Scylla took six screaming men from the ship while their friends watched as they get eaten alive.
The boat moved on through the water, on to the next land, and everyone was screaming at Odysseus for letting those men die. But Odysseus had to do it, he could either lose six men, or his whole crew.