Heraclitus was one of many pre-Socratic philosophers, and he's considered to be the most important and influential. I don't know why, I find him a bit contradictory.
His way of thinking was the result of perception and intuition. He despised rational, logical, conceptual thought. His pronouncements were purposely self-contradictory. "We are and at the same time are not." "Being and nonbeing is at the same time the same and not the same." (I'm totally confused)
He posed two main ideas -
1- The Heraclitean doctrine of "flux" or "Everything is Flux"
This doctrine of flux (or as I understood it "Everything flows") says that the whole cosmos is in a constant state of change. He expressed this view with his famous remark "You cannot step in the same river twice". This remark raises an important philosophical problem of identity or sameness over change. This question doesn't apply just to rivers, but to anything that change over time: plants, animals, it applies to people too, the problem of personal identity - you are not the same person today as you were yesterday.
2- Things change. (Even though I find him contradictory, I do have to agree that everything is in a state of constant change).
Heraclitus wasn't just looking for the primary substance, he believed that everything was constantly changing and he was looking to explain these constant changes or transformations.
He didn't believe change was random, instead, he saw all change as determined by a cosmic order he called the Logos (Greek for "word")
According to Heraclitus, all is fire. Fire, whose nature is to ceaselessly change, is the fundamental substance of the universe, even more than water because fire transforms solids into liquids and because it was always in motion.
He was also a materialist (all objects are physical or material).
I didn't understand him well, in my