/dev/null 2>&1 is where the cron script or command is sending/redirecting the STDOUT and STDERR.
For more details:
Cronjobs are limited to every minute. If you want something to run every 5 seconds, you'll need to write your own script.
/usr/bin/mrtg /etc/mrtg/mrtg.log >> /dev/null 2>&1
Run that in the background, screen process or create your own startup script.