What Shells are they talking about?

Getting model ready to export gCode and I see this reference to 'Shells'.

What 'Shells' are these?
Come to think of it, what is this 'Manifold' thing?
Come to think of it, where would I find this information in the Manual?

Thanks

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"Some people
seem to misinterpret
complexity as sophistication."
(Nicklaus Wirth)

Shells refers to whether or not the inside and outide walls are each continuous. Manifold has a mathematical definition, but in a primitive sense asks "can I draw this without lifting the pencil from the paper. If the answer is yes (the design is manifold) then it can theoretically be 3D printed.

Take this with a large grain of salt. A non-manifold design may actually print very well, and a manifold design may need supports or other "additions" to print properly. Only printing it will tell, although close examination of the slicer output, layer by layer, may reveal what the slicer is complaining about.

In most cases this refers to the STL file content produced from the CAD model.
The STL file provides a surface model of the object. That is it defines a surface which is called a shell. It has no thickness or other properties. And is made from a set of points in space connected by surfaces called facets. A solid cube would be a single shell (exterior) made from the corner points joined by at least 6 facets each of which is a square (side). Normally we use triangular facets so you can have more facets by making a face from two triangular facets.
The term manifold refers to the shell or shells containing an interior. In the case of the cube - if it had 5 square facets it would not be manifold - one side would be missing so you couldn't calculate the inside.
If you have a solid sphere like a marble it will have one shell = outside. If you have a hollow sphere like a football you will have two shells. One for the outside and one for the inside. If there is a facet missing or other errors you can't define whats in the gap between the two shells so it would not be manifold.
If you put your model of the marble inside the model of the ball there will be 3 shells (because there are 2 objects). If the marble shell touches the inside shell of the ball you may have a manifold error because the slicer can't workout the shells. If the shells overlap you will almost certainly get a manifold error. If you join the marble to the ball in CAD then it will be one object with two shells (The inner including the marble).
The slicer can't define the infill for non -manifold models.

Thanks folks. Sorry to be so long getting back.
Thanks for the explanation. I'll get back to the problem.

Wouldn't "5 square facets" indicate a vase structure which is manifold and printable?