These lines tell us that the Green Knight mocked the knights of Arthur’s court for being cowardly and not accepting his challeng
e. King Arthur stepped up to accept the challenge himself, but Gawain does not let his king suffer such and accepts the potentially fatal challenge. By this act, Gawain upholds the of chivalry.
Gawain stepped in to accept the challenged. He chopped off the Knight’s head but it
magically reattached itself back to its body.
A year later he travels to the Green Knight’s lair where it is his turn
to accept a blow from him. When the
Knight strikes, Gawain only to evade the blow.
He goes home afterwards.