Following a suggestion by Edith, this is a forum to post 'jargon' and hopefully get an explanation. Just post the word/phrase you want explaining or reply to one already posted with an explanation.
I don't claim to be an expert in this, and from what I do understand it isn't easy to give a straightforward definition without getting into even more jargon.
In terms of real word RESTful systems you'll find that they treat everything as a URL (actually a URI), and use http methods to manipulate them - http methods are things like GET, POST, PUT, and DELETE.
REST contrasts with an alternative approach called RPC (Remote Procedure Call) - SOAP is an example of this.