What is a multi-layered drawing?

A drawing made in successive layers.

Or a drawing with multiple layers of meaning.

Well, a multi-layered drawing is a drawing that is made on two or more “layers” that are placed on top of eachother.
These “layers” can either be either physical layers, or they can be virtual layers. Physical layers are made by placing/ gluing a piece of transparent paper or a cropped piece of opaque paper over the base layer. using this style, the artist would usually later produce a finalized composition of the image by copying the final image as a whole using various methods, thus eliminating the visible differences in the two layers in the drawing.
The latter type of layering has come with the advent of computer graphics programs. These layers are far more seamless than physical paper layers and can be added or removed from an overall composition without altering the layers below or above it. an example of this are the layers in Photoshop which can be hidden, blended, erased and drawn upon separately from other layers in the composition.

I think it’s like in Iron Man when Tony has a bunch of papers that he puts together to see the first Iron Man suit. If you’ve seen Iron Man, that is.