What is process id in linux?

process id is pid  = this is the id of the running process which is saved in /var/run/

Process ID or PID is the process number associated with every running process in your Linux box. you can think of the process ID like that every child "PID" has a parent "PPID" 

use top or ps aux to list your running process along with its ID numbers and then you can use another Linux command to know the process family tree

pstree PID 

and you should know that INIT process always has PID  #1 as this is the Parent of all other process 

Please let me know if you need much more details regarding to this point

A Process ID OR PID (Process Identification Number) is an identification number that is automatically assigned to each process when it is created on a Linux operating system.A process is an executing (Or running) instance of a program. Each process is guaranteed a unique PID, which is always a non-negative integer.The process init is the only process that will always have the same PID on any session and on any system, and that PID is1. This is because init is always the first process on the system and is the ancestor of all other processes.Listing the contents of /proc with the ls command as follows will show numerous directories whose names consist only of numbers:# ls /proc | less