What is a Mechanical Theorem Prover?

A mechanical theorem prover is a computer program that finds proofs of theorems.

The ideal mechanical theorem prover is automatic: you give it a formula and it gives you a proof of that formula or tells you there is no proof.

Unfortunately, automatic theorem provers can be built only for very simple logics (e.g., propositional calculus) and even then practical considerations (e.g., how many centuries you are willing to wait) limit the problems they can solve.