what is the challenge that the Green Knight offers to the members on the round table?

One lucky Knight gets one swing with an axe. Then, exactly one year later the Green Knight gets to return the swing. Sir Gawain, not wanting to put his king in danger (it's a knight's code thing), accept the challenge. He lops of the Green Knight's head.The Green Knight picks his head up and says, more or less, see you in a year.