Non-disclosure agreements protect the confidentiality of company ideas, processes and technologies, but are not intended to protect unpatented, trade secret, or copyrighted information.
There are three basic ways to keep an idea confidential:
1) Lock it up in your own mind and never share it.
2) Keep it in a physical or electronic form that is inaccessible to others.
3) Apply the NDA to a disclosure of the idea so that the person you are disclosing to has an obligation to keep it confidential.