What are the tests that perform active login attempts?

It means that the scan profile contains vulnerability tests that try to perform any login attempt.

For example, SMB/SSH logins (authenticated scans are not working if this parameter is selected), and any attempt to verify the username and/or password against a system, application/web application, or service.

Read more about how to avoid unwanted login attempts when performing a scan:  https://support.holmsecurity.com/knowledge/how-do-i-avoid-unwanted-login-attempts-when-performing-a-scan-saas