Physics is the natural science that involves the study of matter and its motion and behaviour through space and time, along with related concepts such as energy and force. The main goal of physics is to understand how the universe behaves. In a StackOverflow context physics usually refers to simulating the universe (or some small part of it).
There are several main areas of physics that people commonly try to simulate. Although not an exhaustive list these include:
There are libraries already available for simulating physics in many languages. A list can be found at https://en.wikipedia.org/wiki/Physics_engine