The Roman empire had a very small bureaucratic system, and couldn't spare the resources to set up an equivalent of our IRS. So instead of trying to manage taxes centrally, they privatized the system by selling the right to collect taxes to local collectors. These collectors would be required to pay Rome the amount assessed for the specific province, in exchange for which they'd have Roman muscle backing up their demand for payment from their fellow countrymen. And of course, it was understood that if they managed to collect more than they were required to pay Rome, they could pocket the difference.

This is the context for biblical characters like Zacheus, who was rich because he collected much more than he was required to pay to Rome. This was perfectly legal, but it didn't make him or his fellow tax collectors popular, because they were getting rich by selling out their neighbors to a foreign conqueror.