“a concubine whose name was Rizpah.” It is possible that Saul had only one concubine, because she is the only one named, and it is not clear why she was called a “concubine” and not a wife. Was she a slave? It is possible that Saul did not formally marry her. In any case, she lived a very unfortunate life. She lost her husband and benefactor when Saul died, apparently did not develop any lasting relation with Abner (and in any case he died too), and then her two sons, both by Saul, were executed for Saul’s sin (2 Sam. 21:8).
“Ish-bosheth.” The name is added for clarity; the Hebrew text is just “he.”
“Why did you.” Ish-bosheth challenged Abner because in Eastern culture when a king was killed or deposed the successor claimed the right to his wives and concubines. Saul was dead and Abner had sex with Rizpah, one of Saul’s concubines. Ish-bosheth thought that Abner was positioning himself to claim the throne; and he may have been doing just that in case something happened to Ish-bosheth.
“go into.” In this context, this is an idiom for sexual intercourse.