"A fortiori" is a legal term used to describe a conclusion that is stronger than the one that was originally thought of. Basically, it creates the same conclusion but is considered the primary way in which that is decided.
The use of a fortiori is meant to show that there is stronger evidence that can prove a fact (stronger than the initial evidence that was provided).
A popular example is in saying that if a person is not breathing, then a fortiori the person is dead; the fact that the person is dead is the strongest evidence to prove that the person is not breathing.