The r'uustai is the Klingon rite of bonding, in which two Klingons join together to become brothers, one entering the other's House. Candles are lit and the member entering the Klingon House receives a sash.
Then the words "SoS jIH batlh SoH" are spoken to honor the memory of the participants' mothers – they become one in the House.
In 2366, Lieutenant Worf and Jeremy Aster performed this rite after Jeremy's mother was lost while on an away mission that was led by Worf; he consequently claimed the "right" of r'uustai. (TNG: "The Bonding")