While inspecting the lion it becomes clear that all it is right now is a thin shell with huge gaps in it which is not very printable. What needs to be done is to clean up the mesh so it is closed, manifold, and has a flat bottom suitable for printing.
A solid, printable mesh is closed, or watertight, and manifold.
Closed or watertight is an easy enough concept to get. In cartoons, the characters can blow bubbles of any size or shape they want, but if that bubble gets a single hole it pops. In the same way, the mesh should be one continuous surface no matter the twists and turns it takes.
Manifold is a mathematical term that in ...