text: \] is positive for space-like-separated points \(x,y\). It means that the values of the fields at these points, \(\Phi(x)\) and \(\Phi(y)\), tend to have the same sign. Why? The vacuum is full of random oscillations of fields which is the reason why our measurements (of the values of the fields) tend to have somewhat random outcomes; it's quantum mechanics, stupid. Note that the fields and their time derivatives can't be both strictly zero because that would violate the uncertainty principle.