Determinism is the view that all things in the physical world are determined by previous physical causes, including human action. Two asteroids collide in space because they’re on a path that brings them together. Water falls off a cliff because gravity pulls it down. And you are reading this because some physical process in your brain has led you to browse the Internet and stumble across this article.
Determinism is the result of applying our scientific notion of causality to ourselves, which we intuitively believe to be somehow immune to the tides of causality. But determinism says that we’re a part of the system.