Why we can't find phase relation of a electromagnetic wave for time greater than coherence time if produced by ordinary source?

I am studying temporal coherence and having difficulty in understanding it. My doubt is 1)Why we can’t find phase relationship of a sinusoidal electromagnetic wave for time greater than coherence time. 2) why we can’t say that frequency of wave is not same at all time Is it because of abrupt change in phase ? Or something else 3).What phase relationship actually means in case of same waveform?