Knight (in form of title sir or sire) is a person granted an honorary title of knighthood by a monarch or other political leader for service to the Monarch or country, especially in a military capacity. Since humanity of Northern Kingdoms strongly resembles Medieval Europe, there are a lot of knights in the witcher world.
Knights are also forming units and regiments of army in some states. For example, in Redania they are grouped in cavalry while in Temeria they prioritize fighting by foot. And then there is Toussaint, a land of love and wine within Nilfgaardian Empire, that does not have an army and they are relying on noble knights errants and Ducal Guard.
But what about the knights that choose not to stay at court and stay with ruler? They can become knight for hire, a mercenaries with a noble title, or (and that is happening more often) join some knightly order like Order of the Flaming Rose.