Are you thinking Hush, Hush has nothing to do with the book? After all, the phrase never actually makes an appearance in the book, and it's not like any of the characters run around telling people to keep quiet. Good news—you're not alone in your confusion. In fact, author Becca Fitzpatrick says questions about the title are among the most common she gets (source). Here's how she explains her title choice:
When I was searching for titles, I stumbled across the definition of the word hush in the dictionary. It means "to keep concealed." I thought that was a perfect description of Patch and Nora's relationship in this book. After all, he's keeping quite a few things from her. (Source.)
When this title is considered in line with the other titles of the series—Crescendo, Silence, and Finale—it seems it's also related to music. Hush, Hush is like the subdued intro before things really pick up in future installments in the series.
While music isn't a huge part of this first book, Patch teases out small details about Nora, such as the fact that she plays the cello and that her favorite kind of music is baroque, making assumptions about her character as he does so. And since Nora's also reserved, we can see the title as a little shout-out to her, too.