If you are an aspiring author, you probably already know that you will need to give away some free copies of your book in order to help you get the word out and get reviews. These can be distributed in a variety of digital formats, though there are many reviewers who still prefer printed books. If you are trying to get reviews before your book has gone into print, then you are likely offering an ARC/ARE.
There are several different terms that get used interchangeably, but in general they are called Advance Review/Reader Copy or Advance Review/Reader Edition. In the past, these were printed books, but increasingly they are being offered in only digital format such as mobi or epub. These editions may still contain some copy-editing errors or formatting issues, but for the most part should be as close to the finished book as possible.
When communicating with potential reviewers, you should try to gauge their level of expertise before deciding to use this term yourself. Many book bloggers won’t know the acronym, especially if they are just starting out. But at least now you won’t be caught by surprise if someone asks for an ARC!