Is there a module for site explanation messages? Many sites have text areas which appear to explain things to new users. With an option to hide the message in future. 

I haven't seen such a module for Drupal, does one exist?

I have tried googling but to be honest I have no idea what terminology to use when searching for one.
The closest module I know is the Legal module, this is more for showing the terms of service or similar for users before registering, but it might be close to what you need.