If your book idea is written down, the 'expression of the idea' is copyright protected. This means your characters, plot, argument and conclusion. This applies to all projects that are crowdfunding on Unbound.
The chances that an agent, editor, critique partner, or stranger will:
a. steal your idea
b. execute your idea better than you
c. AND be able to sell it
are next to zero, so it's not something we think you should be worrying about.
If you are still worried, it's worth reading this from Writers & Artists, The Insider Guide to the Media.