System Thinking is a methodology to explore the dynamics of large, non-linear, complex systems that we find everywhere from politics, sociology, to ecology.
Understanding the relationships and interactions within these systems can either help to optimize these systems as a whole or help us to understand why doing so isn't as easy as thought.
A rather simplistic example is the relationship between animals in a biosphere: if rabbits procreate the rabbit population may grow infinitely. Rabbits, however, are part of the food chain of foxes. If the rabbit population increases, so does the population of foxes. The more foxes are searching for prey, the higher the chances that a rabbits get eaten yielding lower rabbit population. Consequently, the population of both foxes and rabbits will be in some equilibrium. See the interactive example here.
If you want to look into some of these system dynamics models, you can use LOOPY. It's a tool to quickly develop own models by drawing entities and their relationships onto a canvas and to and explore the relationships within these models through simulation.