According to the Open Web Application Security Project, an Open Redirect occurs when an application takes a parameter and redirects a user to that parameter value without any conducting any validation on the value.

This vulnerability is used in Phishing attacks to get users to visit malicious sites without realizing it, abusing the trust of a given domain to lead users to another. The malicious website serving as the redirect destination could be prepared to look like a legitimate site and try to collect personal / sensitive information. A key to this is a user just needing to visit a URL and be redirected elsewhere.

In otherwords, you’re looking for a GET request with no user interaction other than visiting a link.