"Ro'n i'n arfer"
Translation:I used to
Ymarfer is the word for "To practice". This is probably what you're thinking of.
Ah in that instance it was talking about "arfer" the noun as opposed to the verb and it makes no sense for this sentence to be talking about the noun.
Not at all. Firstly is the verb "Ro'n"(was), this isn't a word I use much since I use the more traditional form "Roeddwn" and from here you can see that pretty much all of the letters in the middle of the verb have been dropped. Then we have the pronoun "i" which is "I". This is then followed by "yn" or as it is here "'n". This doesn't technically mean anything but it is necessary. Finally we have "Arfer". Here it means "used to" or to connect it to the noun form of "Arfer" it could be translated as "in the practice/custom of". In a sentence this construction might be used like this "Ro'n i'n arfer byw yn Llundain" which would be "I was in the practice of living in London" or more tidily "I used to live in London".