Traditionally the noun knight means someone born of the nobility and trained to fight, usually in heavy metal armor. As a verb, if a king decides to knight you, that means the king wants to make you into a knight.
Real life knights haven’t been around since the Middle Ages, which ended around 600 years ago. These days knight is an honorary title awarded by nations with royal families like the UK. These nations will knight anyone they think is deserving from a rock star to a politician. More casually, you can use knight to describe anyone who acts chivalrous, or bravely. If you are rescued from a burning building, you might call your rescuer, a “knight in shining armor.”