Absolute URL contains complete address from HTTP(Protocol) to domain name and directory/page path that gives the complete information about the file location path.
For example : https://www.techtipnow.in/download.php
Relative URL does not use complete address but contains the file path excluding the domain name. It tells the URL location in terms of the current location. It assumes that the link you add is on the same site and is part of root domain.
For exp: download/index.html