Heraclitus, along with Parmenides, is probably the most significant philosopher of ancient Greece until Socrates and Plato; in fact, Heraclitus's philosophy is perhaps even more fundamental in the formation of the European mind than any other thinker in European history, including Socrates, Plato, and Aristotle.
Heraclitus, like Parmenides, postulated a model of nature and the universe which created the foundation for all other speculation on physics and metaphysics. The ideas that the universe is in constant change and that there is an underlying order or reason to this change—the Logos—form the essential foundation of the European world view.
Everytime you walk into a science, economics, or political science course, to some extent everything you do in that class originates with Heraclitus's speculations on change and the Logos.
Despite all this, and despite the fact that the ancient Greeks considered Heraclitus one of their principal philosophers, precious little remains of his writings.
All we have are a few fragments, quoted willy-nilly in other Greek writers, that give us only a small taste of his arguments.
These passages are tremendously difficult to read, not merely because they are quoted out of context, but because Heraclitus deliberately cultivated an obscure writing style—so obscure, in fact, that the Greeks nicknamed him the "Riddler."
"We both step and do not step in the same rivers. We are and are not."