A premise is not a plot summary — it's the compressed dramatic core that everything else grows from. A good one names the protagonist, their want, the central obstacle, and the stakes, tightly enough that you can steer a whole draft by it.
A vague premise produces a vague book (and vague AI suggestions, if you write with a tool). Sharpening it early is some of the highest-leverage work in fiction. (See how to write a strong story premise.)
Example
'A guarded ex-nurse takes one last criminal job to clear her brother's debt, and it goes wrong in a way that exposes the lie her family is built on.'
See also: How to write a strong story premise