What are Docker image “layers”?

Question

I am brand new to Docker and am trying to understand exactly what a Docker image is. Every single definition of a Docker image uses the term \"layer\", but does not

Answer 1

Per Docker's image spec via The Moby Project:

Images are composed of layers. Each layer is a set of filesystem changes. Layers do not have configuration metadata such as environment variables or default arguments - these are properties of the image as a whole rather than any particular layer.

So, essentially, a layer is just a set of changes made to the filesystem.