Solid drawing refers to the idea that even in a 2D drawing you must consider the character as though they are in a 3d space.
This means giving the drawings more depth by adding shadows, gradients, scale, and perspective. It also means that 2D characters should be able to be drawn from any angle in a believable manner.
An example of solid drawing is Bob McKimson’s Bugs Bunny character studies where you can see believable rotations of the character. It shows that he is aware of the weight, volume and balance of the character