Layer
From Ultrafractal Wiki
A layer is the digital equivalent of the transparent sheets on which objects are drawn or painted for traditional, hand-drawn animation. You can see through the transparent parts of a layer to the layer below.
Each layer can contain a fractal image which can be altered by Masking and Clipping and multiple layers can be combined using Merging into the final fractal image.
Layers are managed on the Layers Tab of the Fractal Properties tool window.
Links
A short introduction to UF layers can be found here
