Modus Tollens is a rule of inference used in propositional logic, used to break up a conditional statement. Basically:
If P, Then Q.
~Q (Not Q)
Therefore, ~P.
If Socrates is a god, then he is immortal.
It is not the case that Socrates is immortal.
Therefore Socrates is not a god.
Modus Tollens is Latin for "the mode that denies", as the ~Q denies the conditional. It is often abbreviated as MT. Don't confuse it with Denying the Antecedent, that's a fallacy, and it just wrong.
Other rules of inference include Modus Ponens, DeMorgan's Rule, and Hypothetical Syllogism.