You have identified an interesting domain name and you want to keep it aside before pre-ordering it, the "favorite" feature is for you. From any domain name, you can click on the heart on the right.
To find all your favorite domain names, go to your account in the "Favorites" section.
To delete them, nothing could be easier, click on the heart again and it will be deleted.
Note: You have put a domain name in a favorite and you can't find it? It is no longer accessible and has just been recovered by a person.