what is FEM?

Answer 1

FEM stand for Finite Elements Method.
It's a method to analyze structures numerically, so the result is an approximation, but if your model is finer, the result will be more precise....
the principle is quite simple, you can model a structure as a sum of a lot of cubes of material (for example, but also rods, plates.... ) and any cube is identified by 8 nodes, and the actions(forces and momentum) cause deformation on the cube and with the sum of all you can determine the deformation of the structure. FEM is used by structural solving softwares on computers(like nastran).