I need to know what the code p1000 means for my Mercury mountaineer 2004

Usually means that the vehicles drive time protocol was not completed properly. The code will have to be cleared and then the vehicle driven for at least 20 minutes without shutting off the engine. If the light comes on during the procedure then it may have a PCM problem but a shop will need to run live data scan to make sure it not a wiring issues.

If you have FIXD OBD sensor, then I would plug that in and clear it out myself through my app on the phone. I've done it to my VW and Nissan Sent a few tiems.