Got to the end of the phrase and have to think about it? Here is the translation: nothing is constant, everything changes all the time. To notice the change, you just have to look long enough. How long is long enough? Depends what you want to observe.
To observe the decay of radioactive elements at the end of Mendeleyev's periodic table you only need a few seconds. To observe social changes, you need hundreds of years. For changes of rock formations, hundreds of thousands of years. So what if some observations take longer than a lifetime? Use fossils, archeology, written records, assumptions, predictions and simulations.
Just like time matters for observing change, mass matters for understanding the physical laws governing matter. Quarks behave differently from objects on Earth or from planets and solar systems.
Will one day be one theory to explain it all?