In Monday's lecture (3.1), there is a plot that shows how the phase is computed that gives a bit of insight. Basically, the relative phase (of the output versus the input) is the "delay" between when the input changes in a certain and when the output makes the corresponding change. Phase lags in feedback systems can be dangerous since if you have 180 degrees of phase lag and you feed this back into the input, you can *amplify* the oscillation (and get instability). We will see this is much more detail around week 7.