The title of East of Eden comes directly from the Bible: "And Cain went out from the presence of the Lord and dwelt in the land of Nod on the east of Eden" (Genesis 22.4.30). It refers to the place where Cain was banished after the murder of his brother. The land of Nod is a place outside of the paradise Eden, a garden to which mankind can no longer return.
Steinbeck models Adam, Catherine, Cal, and Aron on the biblical Adam, Eve, Cain, and Abel. East of Eden is a retelling of this story from Genesis but in the context of the odyssey of an American family. The title reminds us that the biblical first family was forced to reside outside of paradise, but also very close to it. We see this with the Trasks, who are so close to their imagined California paradise but are subjected to the less than ideal, and in fact quite cruel, nature of humanity. They try to create their own paradises, but through various decisions and circumstances, are forever outside of what they seek. Therefore, metaphorically, they all live "east of Eden," just as Cain was forced to.
However, we also must look at what is promised to Cain when he is banished to the land of Nod. Samuel, Adam, and Lee are drawn to the Bible passage in which God gives Cain the opportunity for redemption. This is a key passage for them as it provides them with the hope that they and others can take redemption into their own hands. Various times throughout this story, Steinbeck's characters find the possibility of redemption and this is a theme that is central to the story.