Can someone please explain to me who King Arthur was and what he did?

Legendary King of England although most of what is said of him may be invented. Purportedly ruled with the Knights of the Round Table and united England under the rule of "Right before might". I'd suggest you read https://en.wikipedia.org/wiki/King_Arthur

Separating fact from myth is nearly impossible in his case.

The Battle of Badon was a battle thought to have occurred between Celtic Britons and Anglo-Saxons in the late 5th or early 6th century AD. It was credited as a major victory for the Britons, stopping the encroachment of the Anglo-Saxon kingdoms for a period. It is chiefly known today for the supposed involvement of King Arthur, a tradition that first clearly appeared in the 9th-century Historia Brittonum. Because of the limited number of sources, there is no certainty about the date, location, or details of the fighting.

Just do a Google search for King Arthur. You will find more than enough information.

You haven't heard of King Arthur? Just watch 'Monty Python's the Search for the Holy Grail'. It's 100% factually accurate.