Server Side Request Forgery
The target application may have functionality for importing data from a URL, publishing data to a URL or otherwise reading data from a URL that can be tampered with. The attacker modifies the calls to this functionality by supplying a completely different URL or by manipulating how URLs are built (path traversal etc.).
When the manipulated request goes to the server, the server-side code picks up the manipulated URL and tries to read data to the manipulated URL. By selecting target URLs the attacker may be able to read data from services that are not directly exposed on the internet