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 also form army units and regiments in some states. For example, in Redania they are grouped into cavalry while in Temeria they prioritize fighting by foot. And then there is Toussaint, a land of love and wine within the Nilfgaardian Empire, that does not have an army and so relies on noble knights errants and the Ducal Guard.
But what about the knights that choose not to stay at court and stay with their ruler? They can become a knight for hire, mercenaries with a noble title, or as is happening more often, they may join some knightly order like the Order of the Flaming Rose.