It is in form of an unadorned bangle/bracelet and is usually made of iron. It is one of the five Kakaars or 5K which acts as five external articles of faith in Sikhism. The other four being Kesh , Kanga, Kachera and Kirpan.
A karra is, generally, worn on the right hand but left-handed people can wear it on their left hand too.
In Sikhism and to Sikhs, it has the following significance:
1) Worn on the wrist: Acts as a constant reminder that Our Guru acts through us so we must be careful about our actions. Whatever we do, good or bad; others will judge our Guru or religion through us. We must never bring shame or disgrace to our Guru by indulging in any evil deeds. A mere look at our hands is enough to remind us of our purpose in life, i.e. to act in service of God through service of mankind.