Celts are ancient scotish and irish people. you will have seen them if you have seen the film Arthur.
where you by any chance wearing anything resembling a celtic knot or celtic cross at the time. Celtic knot images
may have been either a compliment or an insult depending on who said it, if it came from someone who dresses in a similar style, probably a compliment, if from some little chav in full burberry gear then an insult.