"2> /dev/null" at the end of a command redirects error messages to /dev/null instead of to stdout.
That way you only see output from the command without seeing any error messages.
/dev/null is also known as the bit-bucket.
When anything is written, or redirected to /dev/null - it is effectively lost.
/dev/null can also be used as an input device, which will provide a stream of zeros.
So you could also completely wipe a usb drive by using:
Bash:
dd if=/dev/null of=/dev/sdx bs=1M
Where /dev/sdx is the device node for the drive in /dev.
Edit: Use the above command extremely carefully! Or better still don’t use it at all!! It’s only there as an example of another practical usage of /dev/null. dd is nicknamed the data destroyer for very good reasons. If you don’t know what you’re doing with it, you can completely bork things!!