A trial by combat is a means by which a party can prove their innocence when accused of a crime in the Seven Kingdoms. In lieu of a standard trial where a lord - or a council of them - hears testimony from the involved parties and makes a ruling, one or all parties may choose the option of a trial by combat. In a trial by combat the accused may represent themselves in combat or, if unable (such as if they are female, injured, crippled, a dwarf, or otherwise incapacitated), may ask for a champion to represent them. The victorious party is held to have had his or her case judged fairly by the Gods (be it the Seven, the Old Gods or the Lord of Light) and has proven their truthfulness in the eyes of the gods. Hence, if the accused party is victorious, they are cleared of all charges.
(Source: oberyndavinci, via fuckyeahgameofthrones)