This is the file in which Squid writes its process ID (PID) number. Squid uses the PID
file in a couple of ways. First, it looks for and reads this file when
starting. If the file exists and contains a valid PID, Squid reports
it is already running under that PID so that you don’t accidentally
start Squid twice. The PID file is also read when you use one of the
-k commands such as squid -k
You probably don’t need to worry about this directive unless you actually do want to run two (or more) Squid processes on the same machine. Each instance of Squid requires a unique PID filename.