It is a specific international rugby championship tournament known as 'The Six Nations' - look up 'Six Nations Championship' on Wikipedia, for example.
Thank you. My ignorance of sport! Does the name need to be capitalised in Welsh or only in English? And I notice that the adjective does not follow the noun in "Hen Wlad Fy Nhadau" ... why?
Well, it probably should be capitalised as it refers to a specific, named championship - we'll probably do that for the next version of the course.
hen is one of a handful of adjectives which usually come in front of the noun being described. Others are sometimes put in front for reasons of style or poetic rhythm. Any adjective coming before a noun causes soft mutation of the noun.