The third section in the traditional annual cycle of Torah readings begins here, with G-d's summons to Avram. The early history of mankind is behind us, and from this point on, the text focuses on the story of the tribe that will become the Hebrew, or Israelite, nation. I've noted earlier that the early history of mankind, as told in the Bible, is a series of displacements; that tradition will be continued by Avram / Abraham and his descendants, the famously wandering Jewish people.
The command "go forth" - [lekh lkeha], or literally "go for yourself" in Hebrew - will be given again to Abraham. Here, he is being told to leave his native land [eretz], his heritage [moledeth], and his family [beth av], for parts unknown - "the land that I will show you". He's not told where he's going, or how he's supposed to know when he gets there. It is a leap of faith.
(In fact, he overshoots his destination and continues all the way into Egypt, a fact discussed at length by Grumet.)
There's some ambiguity about the chronology here, because we were just told at the end of Chapter 11, "Terach took Avram his son, and Lot son of Haran, his grandson, and Sarai his daughter-in-law, the wife of his son Avram, and he set out with them from Ur of the Chaldeans toward the Land of Canaan, and they came to Charan and settled there. And the days of Terach were 205 years, and Terach died in Charan."
Seemingly, then, the narrative first tells of the family's departure from Ur and of Terach's subsequent death in Charan (located in what's now southern Turkey, enroute to the Land of Israel) in order to close the chapter on the lives of the early generations. The text then backtracks to Avram receiving the call to travel. "Perhaps only Abram wanted to travel, and it was his journey that caused the rest of the family to undertake the move." (Steinsaltz)
Grumet takes a different view, holding that Avram has already left his native land of Ur when he receives the call. "Abram's birthplace is Ur Kasdim, and he already left there when his father moved the family at the end of Genesis 11."
To some extent, an understanding of this passage depends on the exact meanings of the Hebrew terms [eretz], [moledeth], and [beth av]. Alter and Grumet disagree over whether the second term means "birthplace" (Alter) or "national identity" (Grumet).
What is certain is that Avram is being called to re-create himself, as he leaves behind him the settled life of cosmopolitan Ur and embarks with his family on a pilgrimage into the unknown.