The "stages" represented here are highly simplified from reality (the "interlocking" and "non-interlocking" barbule stages are basically combined into one, since it's not really possible to tell them apart in fossils) due to making it as easy to understand as possible to creationists.
The caption, written by my partner Jonathan Kane:
Five stages in the evolution of feathers, based on an analysis of feather evolution in a 1999 paper by Richard Prum. Each of these stages in feather evolution has been found on dinosaur fossils except for stage 3, which is known from cretaceous amber.
Stage 1 - Simple fibers: Hollow unbranched fibers, with no barbs or barbules. Found on Sciurumimus albersdoerferi.
Stage 2 - Bundles of fibers: Groups of unbranched fibers, each attaching to a central point. Found on Sinosauropteryx prima.
Stage 3 - Unbranched barbs: Rows of unbranched barbs attached to a central shaft. Found on Sinornithosaurus millennii.
Stage 4 - Barbs and barbules: Rows of barbs attached to a central shaft, which branch further into barbules. Found on Protarchaeopteryx robusta.
Stage 5 - Fully-developed flight feathers: Barbs and interlocking barbules; asymmetrical shape. Found on Microraptor gui.