I recently come across a file with the extension .pid and explored inside it but didn't find much. The documentation says:
A Pid-File is a file containing the process identification number (pid) that is stored in a well-defined location of the filesystem thus allowing other programs to find out the pid of a running script.
The pid files contains the process id (a number) of a given program. For example, Apache HTTPD may write it's main process number to a pid file - which is a regular text file, nothing more than that - and later use the information there contained to stop itself. You can also use that information (just do a
cat filename.pid) to kill the process yourself, using
echo filename.pid | xargs kill