What do you think this means?

"Cool-headed" just means you have to be objective about your book, to the extent that any writer can be. Don't brag about how wonderful it is - how your mother loves it, or how it's going to make you the next J K Rowling or Richard Dawkins. Don't say it's better than Harry Potter or The God Delusion. You might get away with saying something like "It will appeal to readers who liked X and Y" - *if* X and Y are books that the agency sold, because that would show you'd done your homework.