Can someone please tell me what a prom is. I think it has something to do with the computer chip.

programable read only memory. It's the 'chip' you always hear about. ie "I just put a custom chip in my car"


or it's that dance at the end of High School where everybody "gets some"

That's so funny, lol.
Anyways, the PROM is the memory chip that's in the ECM and it has all the calibration about the specific car such as size, weight, engine size, gear ratio, transmission type, emissions, etc... The ECM uses this information and helps the ROM (read only memory) which is the core program to make decisions about the operation of the car. The ROM in turn also has basic instructions that tell the ECM to do something specific whenever is sees a specific input. Confussed?
Example: The engine temperature raises and the coolant temperature sensor detects the change and the ECM receives the signal, according to this change the ROM will command the ECM to change fuel to air ratio or timing advance in accordance to the information stored in the PROM. If a custom PROM is used because the engine was modified then the ROM will react different due to the information stored in the PROM itself.
Now this is my understanding of what it is, and what it does. Hope I didn't confuse you.
Ozzy