Over time, we humans have come up with ways to organize and make sense of complexity. We see an object like the Sun that provides light and warmth, and give that object a name. Then we observe that the Sun appears over the horizon in one direction at first light and disappears in the opposite direction as it gets dark. We have now identified a behavior of that object, and we name that behavior, sunrise and sunset or daybreak and night fall. We now recognize that a cycle is taking place and that it repeats each day, although this behavior changes over the course of the year: daylength varies, as does the location of sunrise and sunset.
To understand why this changes requires a shift in perspective. Eventually, we come to understand that the behavior of the Sun and the behavior of Earth are intertwined, and that these two objects, the Sun and Earth, are part of a system, together with other objects such as the Moon that each have their own behaviors and interactions. These interrelationships cause a lot of the phenomena we experience, including day and night, seasons, tides, eclipses, and in the past century, interference with radio communication!
This systems perspective can apply to all kinds of disciplines: historians, climatologists, wildlife biologists, planetary scientists, organizational leaders, and others have all come to recognize that the objects and behaviors they study, manage, or design are parts of complex systems. Studying systems can help them to understand the whole set of interrelationships rather than just the parts, and to analyze how changing one or a set of parts, or changing the pattern of the parts, can have far-reaching and unexpected consequences in other parts of the system.