/usr/bin/run-parts is the command that is run
/etc/cron.hourly is it's commandline argument.
1>/dev/null is the redirection of the program's stdout (the normal, non-error output) to /dev/null. Had it been 2> it would have ment the redirection of the program's error (stderr) output to /dev/null. Had it been &> it would have ment both, the stdout and stderr.
Hope this sheds some light on the matter...