Ports are usually 8 bit (pins) wide and can be read and written with a single API. Not all the pins of a port need to be connected to the outer world.
P12 describes port 12 and pin number 2 (count starting with zero).
The pin column is related to the physical location of the pin in the chip. You can check the datasheet to verify their location. There are pictures for all different types of packages available.