When Yseut decides to clear herself of the charge of adultery, she insists on having Arthur and his household witness her oath. Arthur is to serve as her "surety." That means that after witnessing her oath, he promises to back up its truth with his sword by fighting anyone who questions it. Yseut chooses Arthur for this duty because she knows he is powerful. So basically, Yseut is like, "You want to mess with my friends, Mark? Bring it."
As Perinis explains to Arthur, "no threats have ever been made to your court, even by a man from some distant kingdom, that have not been properly dealt with" (14.126). In this, Arthur is the exact opposite of the guy Yseut should be able to rely on: her husband, King Mark. But Mark isn't even able to quell the threat of his own barons, let alone his enemies, so Arthur has to take up the slack.
Arthur knows he's a better king than Mark, and he styles himself as a kind of king-instructor to him, chastising him for listening to the "lies" of his barons and making him promise to do better in the future. Arthur just makes Mark look bad, and that's the point: thanks to his example, we see all of Mark's shortcomings as a king.