The department within a corporate entity tasked with
hiring,
firing,
sexual harassment investigation, babysitting, paying, tracking
benefits and, in general, dealing with employees in ways that
management finds tedious or unsavory.
The term indicates that people are resources not unlike office supplies, furniture, buildings, or property.
In the movie "The Matrix" we see the concept carried to its logical conclusion: people bred and harvested for the electric charge they produce as a biproduct of, well, living.