What is Process 644?
The PID of a process will almost always change each time it's run, so '644' is useless to anyone but you. To find the process given a PID, use

ps -p &lt;pid&gt;


Example:

ps -p 1
  PID TTY           TIME CMD
    1 ??        11:09.23 /sbin/launchd


Given the name of the process, you can now close the relevant app correctly — for example, the following tells you that it's Safari, which means you can open Safari and quit it.

ps -p 46239
  PID TTY           TIME CMD
46239 ??        74:37.08 /Applications/Safari.app/Contents/MacOS/Safari