This type of condition will met if specified GET parameter is present and the value is met with the condition. You can use exact, partial or regex values.
How to use it?
If you would like to apply rules if query string contains ref=123 (eg: https://yoursite.com/?ref=123), then simply use the Exact match
If you would like to apply rules if query string contains ref, but would like to accept any value, then use the Regex match, and set (.*) for value.