On UNIX, this is a virtual-file that can be written to. Data written to this file gets discarded. It is similar to the file call NUL on Windows machines.

Key point; When rooting a machine, intruders will often redirect logging to /dev/null For example, the command ln -s /dev/null .bash_history will cause the system to stop logging bash commands.

In layman's terms, it means much the same thing as black hole. Typical usage: if you don't like what I have to say, please direct your comments to /dev/null.

Think of /dev/null as a "black hole." It is the nearest equivalent to a write-only file. Everything written to it disappears forever. Attempts to read or output from it result in nothing. Nevertheless, /dev/null can be quite useful from both the command line and in scripts.

• It discards all data written to it but reports that the write
  operation succeeded.


• It means redirecting both standard output and error to /dev/null


• It prevents the script from displaying anything. like windows "echo
  off"

script /dev/null prevent any message from appearing on your screen. It supresses the messages, byt directing them to the "black hole."

Also, have a look at Why does redirecting 'script' to /dev/null/ allow 'screen' to work while su'ed as another user?

^{Source:Linux Dictionary}