If I understand your question correctly then I assume you are referring to the products of a non-linear device such as a mixer. A mixer output will produce a sum and difference of its RF input and its LO port.
I think he's talking about fourier transforms, and negative frequencies that result. Mabye this will help. It has something to do with phase shifts of the transformed frequencies. If you can't understand this, I'll try to remember the theory I took two decades ago