Systems theory is the study of systems at all levels including couples, families, schools, governments etc. Systems in this theory are self regulating and change according to feedback. There is always a push and give within the system to fit the circumstances. A systems theorist who is a psychologist who was seeing a client would not see the client only but the entire system that the client belongs to, generally the family. Further, the problem would not be the client, but the system would be viewed as the problem.
When looking at situations in our own lives we can generally see how we react to other people and based on what the other person does is how we react. In schools we often blame the kids, or the teachers or the parents but in truth all have play in the problem according to systems theory.
When we are creating our characters we need to keep in mind the different systems that those characters belong to. If the character moves something else will be affected by that move. It may be the antagonist countering the move made by the protagonist.