We think of systems thinking (or systems science) as the science of relationships and perspectives. It crosses the boundaries of many other sciences, particularly biology, ecology, psychology, sociology, management science and many others.
People may think of Peter Senge's book "The Fifth Discipline" when the term systems thinking comes up. Indeed, Senge did a good job explaining causal loop diagrams and systems archetypes. But he only shed light on a tiny part of the systems thinking world. There are many of "campfires of thought" and tools and techniques within systems thinking that are useful and worthwhile. Senge's work focused on one part called cybernetics.
You can learn about systems thinking from any of a variety of websites, books and videos. Here are a couple of suggestions to get started:
"Thinking in Systems" [Donella Meadows]
"Systems Thinking Basics" [Virginia Anderson, Lauren Johnson]
"Systems Thinking Made Simple" [Derek Cabrera, Laura Cabrera]