Protagoras is maybe the most famous of the Greek sophists. The sophists were, in general, teachers of rhetorics who claimed (according to Plato: The Sophist and Theatetus) to have knowledge that can be taught to others so that they would do well in the political life of Athens. Rhetorics was the most important skill for a politician in those days, as we all know.

More philosophically, Protagoras was more of a one-truth man. His book, probably titled "Truth" (though hasn't survived till our days), includes the thesis that things appear differently to different people.

To me, today is a cold day. To you, it might be quite warm. To me, it looks like there is a parrot hopping on the cement. To you, it looks like there is only a plastic bag. (This really happened when I was in Portugal some years ago, and it WAS a plastic bag. I think.)

And thus Protagoras is known for his statement "Man is the measure of all things."

As Plato notes (in Theatetus), the "Truth" of Protagoras falls into a regress. If his truth is that everyone has their own truths, it is only his truth. How can he claim that the truth is that there are no interpersonal truths? Plato has also other undermining theses (though less convincing ones), for example that according to Protagoras, a pig knows the world as well as we do.

Albeit the regress, relativist claims have been popular until our days and even now that scientific realism has a strong support. (Later relativism shall be discussed at lenght on the Scepticism topic later on.)

To Plato, this kind of conceptions were the worst possible (with the possible exception of Democritus's atomism). Thus he decided to found knowledge in a more Eleatic way. Let's discuss this on Plato's Epistemology topic.