What does Genesis 35:26 mean?

In this passage, all of Jacob's sons are recorded, grouped by their birth mother. Zilpah was Leah's personal servant, given to her by her father Laban (Genesis 29:24). Leah married Jacob only because her father, Laban, tricked Jacob, who wanted to marry Rachel (Genesis 29:25–28). Jacob's lack of affection for Leah led to a rivalry with Rachel (Genesis 29:30–31). This expressed itself, in part, in a competition about bearing his children. As Rachel did with her servant, Leah brought Zilpah into the marriage as a servant-wife to bear sons on her behalf. According to legal customs of the times, these would have been considered Leah's children.

In this way, Zilpah bore Gad (Genesis 30:11) and Asher (Genesis 30:13).

Aside from Benjamin, recently born to Rachel in the Promised Land of Canaan, Jacob's other 11 sons were all born when he lived and worked with his uncle and father-in-law, Laban, in Paddan-aram.

The list of sons is now complete; Jacob will have no others in this generation. These 12 will begin to fulfill God's promise to grow into multiple nations as they go on to have sons and grandsons of their own.