What challenge does the Green Knight make to the court?

The Green Knight  explains that he has come to this famously valiant court to play a Christmas game. Whoever agrees to play this game will be allowed to strike the Green Knight on the spot, in the middle of the court; in exchange, the Green Knight will strike a return blow upon the volunteer a year and a day hence. None of the court volunteers as the game seems to imply certain death for whomever plays; the stranger ridicules them all for Camelot's supposed bravery.