So I guess it is best to remember this as, 'They are liking chocolate' in terms of the translation?
'r is a form of the Welsh word for "the" -- see the lesson notes for the unit "The": https://www.duolingo.com/skill/cy/The
'n is a grammatical particle which doesn't have a translation in English. It's used to link a form of bod "to be" such as dw i or maen nhw to another verb such as hoffi. It's not used with eisiau or angen but with pretty much all other verbs.
For more information on those lesson notes in general, including how to access them from a mobile device, have a look at https://www.duolingo.com/comment/17638579 .