Among other philosophical influences the most
significant that Plato was exposed to, were those of Heracleitus and his followers, who disparaged the phenomenal world as an arena OF CONSTANT CHANGE AND FLUX, and of the Pythagoreans, with whose metaphysical and mystical notions Plato had great sympathy.

according to what I have learned in

The phrase should be "Ta panta rei"
This is the sound bite for the philosopher Heraclitus.
He lived around 500 bc. He said "you cannot step in the in the same river twice" the implication being that it "everything is constantly flows"
Heraclitus, however, thought that the primordal element was fire.

Several philosophy 'text-books' will tell you all about Heraclitus.

I recommend Bertrand Russell's History of Western Philosophy and the documentary comic books "Philosophy for beginners" by "Richard Osborne" and "Introducing Philosophy" by Robinson and Groves.