2 Samuel 5:13 And David took him more concubines and wives out of Jerusalem, after he was come from Hebron: and there were yet sons and daughters born to David.
Female slave who functioned as a secondary wife and surrogate mother. The Hebrew word for concubine (pileges) is a non-Semitic loanword borrowed to refer to a phenomenon not indigenous to Israel. Babylonian and Assyrian law codes regulate primary and secondary marriages more specifically than do the Old Testament laws. Exodus 21:7-10 has been appealed to as regulative of some aspects of concubinage, but that only implicitly.
Concubines are mentioned primarily in early Israelite history during patriarchal times, the period of the judges, and the early monarchy although some later kings also had concubines. While concubines did not have the same status as wives, they were not to be mistreated (Exod 21:7-10) nor could they be violated by other males (Gen 35:22) with impunity (Gen 49:3-4). They seem to have received higher status if they bore sons, or at least they are remembered by name (Gen 21:10; 22:24; 30:3; 36:12).
The sons of some concubines were treated as co-heirs with the sons of wives. Was this facilitated by the wife accepting and naming the child as her own, or was the father's act of "adopting" the son required? Paucity of information prevents us from answering this definitively. In at least one case the inheritance potential of the concubine's son seems to present a threat to the primary wife and her son (Gen 21:10). Abraham eventually gives the full inheritance to Isaac, and only gives gifts to his concubines' sons (Gen 25:6).
The story of Judges 19-20 suggests that the terminology used of relationships in a regular marriage are also used in a concubinage relationship. The man is called the concubine's "husband" (19:3; 20:4) and the woman's father is referred to as the man's "father-in-law" (19:9). Some evidence suggests that royal wives (concubines?) were inherited by succeeding kings (2 Sam 12:8). Thus approaching the royal concubines (2 Sam 16:21-22) or even requesting the king's female attendant for a wife (1 Kings 2:13-22) can be understood as the act of one attempting to take the throne away from its designated occupant (1 Kings 2:22).
The practice of taking concubines as "wife" was used to provide a male heir for a barren wife (cf. Gen. 16, 35, 36). In addition, the practice provided a social safety net for poor families who could sell their daughters in dire times (Exod 21:7-10; Judges 19:1). It seems plausible to suggest that the practice of taking concubines was perpetuated to meet the sexual desires of the males and/or to cement political alliances between nations. Nevertheless, the paucity of sufficient internal data requires dependence on comparative ancient Near Eastern evidence for these conclusions. Multiplying children through concubines would not normally complicate the inheritance lines, but would increase the available family workforce and the family wealth.
I hope you find this helpful.