Solid drawing is when you draw thinking about how the character would exist in 3D space. Rather than the character being made of squares and circles, cubes and spheres can be used. This is also good because it is easier to warp the drawing to show convincing perspective, and it can stop the drawing from looking flat.
Solid drawing also makes creating a turnaround a lot easier.
Turnarounds are used in production boards and production bibles. They can be used as a tool to create 3D models of the character either physical or digital. They are also good to showcase the character and to judge if they animate well as a turnaround is the most basic and important animation to do with a new character.