The emblem of the knights of the round table worn round the necks of all the knights was given to them by king arthur as part of the ceremony of their being made a knight.
The orders dominant idea was the love of god, men, and noble deeds.
The round table was a physical table, but the highest order of chivalry at the court of king arthur. It's members were supposedly the cream of the british military who followed a strict code of honour and service.
Accounts differ about the origin of the round table at which arthurs knights met to tell of their deeds and from which they invariably set forth in search of further adverntures. The norman chronicler wacce was the first to mention it in his roman de brut of 1155. There he simply says that arthur devised the idea of a round table to prevent quarrels between his barons over the questions of precendence.