Null

The null device is located in /dev/null usually and discards any data written to it. It's sort of a black hole that eats everything that comes close to it. Unwanted output in shells can be redirected to /dev/null:

In case of a crontab entry output can be discarded causing no mails to be sent:
*/5 * * * * /usr/local/bin/fetchmail > /dev/null 2>&1