Why is there a proverb in Shaanxi that "ancestors came from the west and ancestors came from the east"?

Shaanxi's "Collection of Folk Songs" and "Pilgrim's Progress-Thinking of My Grandma" are all sung. Who will tell us about this history? The flower in Shandandan is blooming, red and gorgeous, and the kindness of our ancestors cannot be forgotten! I can't forget my ancestors from the west for generations! My grandparents in the East laid eggs and married! The cave is beautiful, the waves on the heatable adobe sleeping platform, the ancestors in the west, grandparents in the east! One gives me a first name, and the other gives me a last name! My parents' kindness will never be forgotten! Never forget!