Systems thinking is a way of looking at the world that focuses on noticing the ways in which things are related to each other. Understanding the systems in our lives helps us make sense of the world around us, and helps us find better strategies for making changes. A system is a collection of parts that interact with each other to function as a whole. That's a pretty general definition, and in fact, there are systems everywhere. The human body is a system, and it also has systems within it, like the nervous system, the respiratory system, the digestive system, etc.
Atoms are systems, as are solar systems and galaxies.
There are social systems such as families, schools, churches, and governments. For something to be a system, it has to matter how the parts are arranged. If you have a bunch of machine parts on a shelf, that's a collection of parts, but not a system, because they don't interact. But if you put all the parts together to build something, then you've made a system, and the way it works depends on the interactions between the parts. And there are systems all around us.