A man in full armour was unrecognisable so each man wore a distinctive coat, by which he could be recognised, over his armour. This was called his coat "of arms".
As no two men in the same region could wear exactly the same coat of arms, these "arms" were soon used separately as personal symbols -- and especially to mark their owners' possessions, because few people could read in those days.
The Hundred Years' War was a series of conflicts waged from 1337 to 1453. Each side drew many allies into the war. It was one of the most notable conflicts of the Middle Ages.
The second son gets a crescent, the third a mullet, the fourth a martlet, the fifth an annulet, the sixth gets the fleur-de-lys, the seventh a rose, the eighth a cross moline and finally a ninth son gets a double quatrefoil.