Whenever I've heard the instrument discussed, fiddle = violin. Wikipedia
A fiddle is any bowed string musical instrument, most often the violin. It is also a colloquial term for the instrument used by players in all genres, including classical music. Fiddle playing, or fiddling, refers to various styles of music.
I don't play the violin (I play the cello), but my brother started taking classical violin lessons when he was in middle school. He had the idea that he wanted to learn how to fiddle when he was in high school, I think, but always seemed to have more trouble with it than with his usual classical playing style. Playing classical music generally involves learning to read music and following a set of well-defined rules. On the other hand, since fiddling is primarily folk music, the rules are less defined and there may or may not be any music to follow. It always seems to me that it requires at least a little bit of improvisation, not to mention complex fingering.
I can say from personal experience playing the piano (for most of my life), I can read music like a champ and have at least a fair amount of technical skill. But I really struggle with music outside of the classical genre, and even have a bit of a hard time with 20th century classical. Composers like Debussy require a more free-flowing style (and often require that your hands work independently of each other), and I'm just not very good at it.
So I find it somewhat ironic that the poster's classical violinist teacher looks down on fiddling, when I've always thought it was probably harder than classical style violin playing. It certainly isn't easier, and often requires a great deal of technical skill.