Mesh generation is a vital step for any simulation be it structural, NVH Or CFD. It involves breaking the entire model or domain (which is the geometry) into small elements with certain qualities. A more technical term for this process would be discretizaion.
Why would you need this in the first place! Well any geometry can be broken into infinite number of parts and if you are going to apply governing equations on them for solving a case, solver would go crazy as it'll have to solve them for Infinite number of inputs.
This is tackled by dividing the geometry into a finite number of chunks or pieces called elements. And this process is termed as meshing. There are basically two types of mesh used ; 2D or shell mesh and 3D or solid mesh.
Now a simple guess would be 2D mesh can be used for 2D geometries and 3D for solid geometry right! But in actual practice for certain 3D components you can very well go for shell mesh or 2D mesh. Actually for plastic and sheet metal components which are having a uniform thickness a mid plane is extracted out of the solid model. Mid planing or mid surface generation is a very delicate process in itself, it requires a CAE engineer to have basic grasp of CAD modelling like surface generation, boolean operations etc. After that the crated mid surface is meshed with 2D elements and thickness is assigned to them in such a way that half of the value goes in upper side and half in the lower side.
This approach saves precious resources in carrying out the simulation as 3D mesh is always heavier for the solver than a 2D or shell mesh.