Set of concepts that attempts to explain complex phenomenon not explainable by traditional (mechanistic) theories. It integrates ideas derived from chaos theory, cognitive psychology, computer science, evolutionary biology, general systems theory, fuzzy logic, information theory, and other related fields to deal with the natural and artificial systems as they are, and not by simplifying them (breaking them down into their constituent parts). It recognizes that complex behavior emerges from a few simple rules, and that all complex systems are networks of many interdependent parts which interact according to those rules.
I found the Cynefin framework helpful in understanding Complexity Theory (see link and paper). In terms of school culture (a big topic in itself), think of the many elements that make up the culture of the school (leaders, teachers, students, parents, funders, departments of education, politicians, training providers etc). Now imagine that you want to do something relatively simple (if only it were), like improve student learning outcomes. Simple logic would have you identify the causes of better learning outcomes and apply pressure there. But in schools, where would you start? Leadership? Teachers? Money? Student motivation? Parental support? The answer is of course non of these levers on their own will impact student learning with any degree of certainty because the whole system needs to be moved. The more elements there are in the system the more complex it becomes. For example if all your students were English speakers from a similar cultural background, you don't have to think about language and culture. The moment you throw in culture and other first languages into the mix, you've added complexity to your system. Hope that helps!
https://www.youtube.com/watch?v=N7oz366X0-8
Article Red Dirt Thinking on Education: A People-Based System