"The elder shall serve the younger" is a quote from the Old Testament of the Bible, referring to Isaac's twin sons, Jacob and Esau. In Genesis 25:19-34, the reader learns about the relationship between the two, even before they were born—as they struggle in the Rebekah's womb. Verse 23 (NIV) reads:
The Lord said to [Rebekah]: Two nations are in your womb, and two peoples from within you will be separated; one people will be stronger than the other, and the older will serve the younger.
Isaac loved Esau, but the scriptures note that Rebekah loved Jacob. As with many cultures, the oldest son was expected to inherit his father's wealth and power. It is with Rebekah's help, as well as Esau's lack of wisdom, that Jacob was able to wrest from Esau his birthright ("the right to be recognized as firstborn with authority over the family"). Later, through trickery, Jacob also gains Isaac's blessing (that was rightfully Esau's)—allowing Jacob to inherit all that belongs to Isaac—most importantly God's blessing, promised to his grandfather, Abraham.
The well-known story is that Esau comes home extremely hungry. He offers his birthright to his younger brother Jacob in exchange for some stew. Years later, as Isaac's health is failing, Jacob (with Rebekah's help) tricks his father (who is blind) into believing that Jacob is really Esau. Isaac gives his blessing (believing that he is blessing Esau) to Jacob. While Esau was born first—and the elder—Jacob becomes master over all that is his father's, and Esau becomes, in essence, subservient to Jacob. Jacob—and his people—are stronger than Esau; Esau loses his right to lead the family, essentially serving his brother.