What is a tagline?
Taglines are those quotes or lines of text on the cover of a book that isn't part of the title, summary, or author name. In branding we call it a slogan. One of the most famous ones is probably McDonald's. I don't even have to write it out for you to remember it.
Books have them as well! Here are some examples from popular books:
"Winning will make you famous. Losing means certain death." - The Hunger Games by Suzanne Collins
This one explains the premise of the book without any fuss. To the point but engaging.
"A world at stake. A quest for the ultimate prize. Are you ready?" - Ready Player One by Ernest Cline
Fairly generic but still explains the plot pretty well. It get me excited anyway, but that might be because I've already read the book and love it...
"One choice can transform you." - Divergent by Veronica Roth
I'm a bit undecided if I like this one or not. I've seen the film so I understand what it's hinting at, but it's still not really saying anything.
"Meet Celaena Sardothien. Beautiful. Deadly. Destined for greatness." - Throne of Glass by Sarah J. Maas
Generic as they come. Exchange the name for anyone else's and it could be applied to any story. I disliked this book quite severely, but even if I had liked it my view of the tagline would still be the same.
Don't think taglines are what's going to sell your book. It might help, but your cover could do just as well without it as with it. It's all a matter of taste really. If you have the type of summary at the back that could be helped with a foreboding message, then go for it. If you can't seem to figure out something that would work, then skip it rather than wing it.
"What makes good taglines" is probably what you're wondering now.
There's no one answer, but here are some pointers on what most likely would make them more interesting:
Use a good quote from the story itself that might come to mean something important in the future (think "winter is coming" or similar)
Reflect the theme/stake/core of the story
Highlight something unique about the story
Create a sense of tension without being melodramatic
Don't mislead a reader by making a funny tagline when your story is actually serious and dark
Give it a nice flow, possibly a bit poetic, but don't be generic
Clever or funny (think a bit Pratchett)
Inspire curiosity by causing the reader to ask questions, or something odd that they'll want to find out what it means
It'll be close to impossible to create a tagline that fill all these boxes so just consider what type of story you have and work with that. What is your style? What do you like? What have others in your genre done before? What would fit?
The last thing you can do is to also write several of them and then give them to people you trust and see which ones work for them, or if they can suggest improvements or even come up with stronger ones!