What is purfling?

Purfling is the narrow inlaid edge of the violin plates.

They are made of laminated straps of wood in a contrasting color to accentuate it visually.
Cheaper violins, mass-produced instruments often will have a plastic inlay, or even a painted one.

Sometime the creativity of the luthier goes a few steps further.

This is quite beautiful, however this will affect the vibration of the plate, (and thus, the sound quality).