That was a VERY generic question. Care to elaborate?
It makes simulations of physical behavior of objects. It is used to predict the strength and rigidity (among much other properties) of mainly mechanical components.
It divides a solid model into many incremental pieces and run a series of increasingly complex equations on them until it reaches a certain level of fault margin, upon which it stops and the result can be visualized.
As for sources, PTC and affiliates are likely more than happy to sell you courses for a mountain of money. And there are books: