what is the 0x7DF PID code?

I've bought a Serial CAN Bus Module v1.4 and I'm trying to get data from my car OBD port.
I've run the Arduino examples and couldn't get speed and rpm.
I don't know what is the correct PID and how can I use them.
for example, I need to find my car PIDs what is the 0x7DF PID in the examples codes?