This feature allows you to pre-approve content before it's added to the rotation database.
In Rotation - Preload 1, you can mark the most promising thumbnail from a gallery. For example, if you added 10 thumbs to a gallery, the script will test all of them to find the best performer. However, if you believe one is clearly superior, you can click it twice to highlight it with a yellow frame. This marks it with cast priority. This doesn't stop rotation for the other thumbs—it just means the priority thumb will be tested first before the others.