They were called this way after being granted with the title of knighthood by a monarch or a bishop for services to them, especially in a military capacity.
It was conferred to mounted warriors and was considered as a class of lower nobility. They served to a lord and got paid with land holdings.
By the end of the medieval age, the new methods of warfare made the knights obsolete for the battlefield, but the title stills remain to these days as a recognition to the achievement of somebody.
