I wouldn't say there is a specific definition for what a good description is, however there are definitely some things to consider when writing one.
The first thing to consider is your spelling and grammar in your description; if you have a typo or a mistake in your description, this may put people off. Remember that your description is a reflection of your book, so making it perfect will give your book a good first impression.
There is a certain art in writing descriptions as you have to keep a balance of telling the potential reader what the book is about yet still keeping them curious. I suggest telling your reader four things: the names of your characters, the general idea how the story STARTS (don't give them too much information about what goes on in the story), a vague idea of the plot twist without giving any real information and the location/era that the book is written in.
Sometimes rhetorical questions can work in your favour, however they can sometimes come across as forced or cheesy. If you're using a question that you've seen in around twenty book descriptions on Wattpad, it's probably overused. Questions like 'Will it end in love?' are slightly cliche and have been used in so many book descriptions.
If you're really not sure about your book description, read it to someone and ask if they are intrigued about the story.