I have been given the following equation to show the difference between coherence and non-coherence in optics and how the third term represents the interference between two optical pulses. Can someone explain what the terms represents and how this equation changes when the two signals are the same continuous data signal but $\vec{Z}_2$ is now $\vec{Z}_1$ delayed by $\Delta t$?
\begin{equation} I(t) = \left| \mathbf{\vec{Z}}_1(t) + \mathbf{\vec{Z}}_2(t) \right|^2 = \left| \mathbf{\vec{Z}}_1(t) \right|^2 + \left| \mathbf{\vec{Z}}_2(t) \right|^2 + 2 \left| \mathbf{E}_1 \right| \left| \mathbf{E}_2 \right| \cos\phi \label{eqn:coherent} \end{equation}