In the computing world, a process is a single instance of a program that is currently running. You can view currently running processes by bringing up Task Manager and hitting the "Processes" tab. You can also go to the command line and type in "tasklist".
You'll notice the column for PID. That stands for process ID and it's sort of a take-a-number scheme that the CPU hands out to processes in order to keep track of them. The PID is listed in Tasklist by default but not in Task Manager. To view the PID in Task Manager, open it, click on "View", click on "Select columns"
and check off "Process ID" and then click "OK".