In the Klingon legal system, a warrior standing trial or challenging the High Council could appoint a cha'DIch, or "second." Since the accused warrior is denied combat privileges, the cha'DIch must physically defend him.
Kurn served as his brother Worf's cha'DIch when Worf challenged the Klingon High Council in 2366. When Kurn was injured in an assassination attempt by Duras, son of Ja'rod, Jean-Luc Picard became Worf's new cha'DIch. (TNG: "Sins of the Father")