In medieval times, a knight was a man of noble birth, who served his king or lord in battle.

If someone is knighted, they are given a knighthood.

A knight is a man who has been knighted.

In chess, a knight is a piece which is shaped like a horse's head.

If you refer to someone as a knight in shining armour, you mean that they are kind and brave, and likely to rescue you from a difficult situation.