Purple fringing, or bichrominance as it's more correctly termed, is a form of chromatic aberration caused by the over-excitation of the pixels on the sensor in the camera. The effect can occur anywhere within an image but it's most common at the edges of high contrast areas, especially when a dark element is strongly backlit, such as branches silhouetted against a blue sky. The Defringe filter selectively adjusts these areas to remove the color fringing.
This filter can be applied as a non-destructive, live filter. It can be accessed via the Layer menu, from the New Live Filter Layer category.
On selecting the Defringe filter, it will default to settings which remove the purple fringing, as this is the most common type. This can be manually adjusted as necessary in the dialog, or by sampling the fringe color on the page to remove it.
The following settings can be adjusted in the filter dialog: