How can someone describe their book idea without revealing any details about the plot?

Follow this format and see what you get for your descriptions.

First start with the WHO.

Who is your main character. Name them and describe them in some brief way so the reader gets the gist of what kind of person they are.

Describe the PROBLEM.

Every fiction book has some kind of problem that must be overcome. You can't leave this out because you are afraid of leaking plot elements. Just keep it simple.

Finally we have the STAKES.

What might happen if the above problem isn't overcome? High stakes and a tough problem together are what catch a potential reader’s interest.

It takes practice and many many revisions, but that is the basis to use for both book proposals and sales blurbs.

Good luck in your writing