The cha'DIch is a position within the Klingon legal system and is essentially a warriors 'second' during a trial. This is because during the trial, the combat privileges of the accused are denied, the cha'DIch must physically protect him.
In 2366, Kurn served as Worf's cha'DIch when he challenged the Klingon High Council. When Kurn was later injured during an assassination attempt by Duras, Jean-Luc Picard took over the role. (TNG episode: "Sins of the Father"; TNG eBook: A Sea of Troubles)