In the early Middle Ages, poets and story-tellers were telling a lot of different stories about knights and ladies and their adventures. But how did all these stories fit together? People wanted all of these stories to be tied together somehow. So the poets came up with the idea of the Knights of the Round Table.
It’s a framing story, like the framing story in the earlier Odyssey, the Mahabharata, or the Arabian Nights. A poet named Wace wrote the earliest written description of the Round Table, about 1150 AD.
The table was at the court of King Arthur, and it was round so that all his knights could sit at it without anyone seeming more important than anybody else: they would all be equal to each other (though not to women, or peasants, who couldn’t even sit at the table).