The one before the Cat is the upstream O2 sensor.  It measured the percent
of misfire (or unburned fuel) going to the cat..  The one after the Cat is
the downstream O2 sensor.  It measures the how efficiently the Cat is
working.

I believe it was after 1996 models when OBII standards came in that mandated
an error message if the sensor was unplugged.

It is also possible that a previous owner has put a resistor in the harness
and unplugged the sensor so the computer would read a more desirable range
(due to the resistor), then allow for a richer mixture.