1998 Mazda Millennia check engine light

1998 Mazda Millennia mileage: 72,000. Yesterday, I got a check engine light. The local AutoZone diagnosed the P0173 code to be "fuel trim". They couldn't elaborate on what that might mean. I just bought the car and to my knowledge the injectors haven't been cleaned. I did have to replace an O2 sensor a month ago. I just read on the web that some cars require a mechanic to disconnect the battery when changing the O2 sensor to reset the long term fuel trim. I remember having to reset the radio after their work. Is it the injectors? What am I missing?