A vignette is a photograph whose edges fade out gradually to white or black. In photography, vignetting can be an unintentional result caused by optical limitations in the camera lens. As an effect, it is frequently used for portrait photos, usually in an oval or other shape.
A vignette effect is easy to apply in Fhotoroom. There are 2 methods to create them in Fhotoroom, the first and simplest is to use the built-in Styles, the second method is by making a selection before loading the Vignette filter.
In the video below you will see how method 2 is applied using 2 selections.
Vignette is located under the Filters menu option Filters > Vignette... and is available in 8 and 16bit.