What a purfle is depends on what it's applied to: On clothing, it's typically a ruffle but can be embroidery. On musical instruments such a viol, it's an inlay of differently colored wood. Furniture, it can be that as well or breeding (line of beads) or cording. In heraldry, a border of one of the furs or sometimes spaced gold studs. And so on -- the main point being it's a border and it's decorative. It's one of those words that, in the clothing sense, sounds much like itself. It entered the language around 1300 as purfilen, from Old French porfiler, to make or adorn a border, from Vulgar Latin *prōfīlāre, from Latin prō-, forth + fīlum, thread -- thus showing the clothing is the original sense.
Mary was all set for the dance, her hair neatly braided, wearing a blue gingham dress with a purfle that just swept the floor.