“Ahinoam of Jezreel.” This is the “Jezreel” of Judah, not the Jezreel in the Jezreel Valley. This Jezreel is in south-central Judah, not far from Maon, Ziph, and Carmel (cp. Josh. 15:56; 1 Sam. 25:43; 27:3; 30:5; 2 Sam. 2:2; 3:2; see commentary on Josh. 15:56).
“and the two of them were his wives.” The need for a royal heir who could continue the dynasty was such that most kings had more than one wife. In this case, David’s having more than one wife was important because Abigail’s only son, Chileab, David’s second son (2 Sam. 3:3), seems to have died as a child. He is never mentioned as being part of the life of the royal family. David married more wives later.