What is a "PIT file"?
PIT stands for Partition Information Table. A PIT file is basically a set of instructions defining the phone partition layout. You should use a it if you want to re-partition the phone with Odin. 

You only need this file when repartitioning the device and you don't need to repartition it unless you mess up the partition table to begin with, or when downgrading/upgrading.

There are tools (commercial) that can genarate PIT files using ADB commands.