The kara is a plain iron or steel bracelet worn on the wrist. Not gold, not silver, not decorated. Iron is deliberate: it is the metal of tools and weapons, not of ornament.
It is worn on the hand that acts, and the usual explanation given is that it is a restraint, felt when the hand is raised.
It is the most commonly worn of the five kakars by Sikhs who keep none of the others, which makes it the practical marker of identification worldwide.
Bhai Randhir Singh fought and won the right of Sikh prisoners to keep the kara in British Indian jails, where it was confiscated as metal.
It has been the subject of school uniform litigation in Britain and elsewhere, and in 2008 the English High Court held in favour of a schoolgirl excluded for wearing one.
It is worn on the right wrist by most, and the reading given is that the hand which acts is bound. It is also read as the unbroken circle and as the relationship that circle stands for.
Iron rather than gold is the point. The kakars are deliberately common materials and a Sikh institution that supplies them supplies iron.
The argument Bhai Randhir Singh made in jail over the right to keep it established the principle in Indian prison practice, and the same argument has since been made in schools, in workplaces and in courts abroad.