That there's fragments of history behind the legend of Arthur.
A long time ago the only iron they had access to in Britain was bog iron – literally clumps of mud pulled from bogs. They would use them to make iron weapons but the quality of the metal was poor.
"pull the sword from the stone and you will become king of Britain" – this means if you learn to get iron from iron ore (stone) you will get weapons good enough to make your people conquerors.
"a magic sword from a lake"
Heard of quenching? at some stage someone was forging swords with their new iron and accidentally dropped it in some water (or stuck it in because he was tired of waiting for it to cool down) and to his surprise found the new sword seemed "magically" stronger than non-quenched swords.
Remember; millennia ago history was largely (completely, sometimes) passed down orally from generation to generation and changed in the process.
The legend of Arthur that we have now is in fact the relicts of actual historical events so old there's nothing left of them except the legend…
Not all legends are true; but some may be fragments of oral history.