Later on, a knighthood became an honour that could be conferred by the British Sovereign on civilians. The recipient's name is preceded by Sir. It is not a hereditary title. The female equivalent of a Knight is a Dame.
Baron is the lowest rank of the British peerage, and is a hereditary title.
The Battle of Agincourt was fought in 1415, and was one of the defining battles of the Hundred Years War.
When Lord Marchmain speaks of the larger honours, he is referring to the fact that he is the Marquess of Marchmain, a title inherited from his predecessors. Marquess is the second highest rank amongst peers of the realm, second only to Duke.
The Georges are the Kings: George I, II, III, IV, who ruled Britain between 1714 and 1830.