I can't give you the academic answer, but I can tell you what it means in practice.

Rest is a standardized way of structuring url's and http verbs.

For instance:

1) To get all "posts" from a blog, you send a GET request to www.whatever.com/posts

2) To read a single post, you send a GET request to www.whatever.com/posts/<id>

3) To create a single post, you send a POST request to www.whatever.com/posts

4) Finally, to get all comments from a post, you send a GET request to www.whatever.com/posts/<id>/comments