What is a relative path?

Answer 1

absolute path: http://www.sitio.com/assets/css/file.css
relative path: /assets/css/file.css
(if you are in the root)

Answer 2

a relative path is a path relative to the folder it is called from, for example say A.PHP and B.PHP are .PHP files in the same folder, if A links to B the path is just the filename B.PHP.
If A was in a folder that contains a folder 'X', and B was in that folder, the path from A to B would then be X/B.PHP : first the folder, then the file name. These paths are relative in that they only work in that one folder, anywhere else 'X/B.PHP' wouldn't be a valid path.