Why are the figures on the urn called a "leaf fringed legend"in Keats' "Ode on a Grecian Urn"?

I believe Keats uses this phrase to emphasize that he does not really know what the scenes on the urn were meant to represent.

In the next few lines, he twice asks whether the figures on the urn are mortal or gods.  So it is important to note that the words you mention are part of a question.  He is asking if the leaf fringed (because there are trees and such painted on the urn) figures are part of a legend and if so, what legend.

So he uses these words (and the questions around them) to show that he is really not sure what the scenes on the urn are meant to represent.