How does a non disclosure agreement work?

Non-disclosure agreements normally have more psychological than legal value. Further, they must be supported by "consideration" (the exchange of something of value.) If you are getting nothing, the agreement may be unenforceable. However, if this is an employment related situation, remember that in an "at-will" employment state like California normally you can be fired for no reason that is not discriminatory against a protected class. A review of the document would give you the best analysis.