From Halopedia, the Halo wiki
A PQI is a lie detector used primarily by the Colonial Administration Authority police. Because it is often referred to as a "pop quiz", it is speculated that it stands for "Pop Quiz Interrogation" or something similar.
It uses a hand plate and most likely an eye scanner to detect if a subject is lying. It is possible to trick the PQI into believing that the subject is telling the truth, as done by Janissary James.