Short answers, pulled from the story.
Protagoras argued that each person's own experiences and history shape their perception of what is true. He illustrated this with temperature: one person may feel cold while another feels hot, and there is no absolute evaluation outside the individual perceiving it. His lost work Truth, where the statement appeared, seems to have been arguing for individual relativity rather than a blanket denial of reality.
Protagoras was a pre-Socratic Greek philosopher and rhetorician born in Abdera, Thrace, opposite the island of Thasos, around 490 BC. Plato credits him with inventing the role of the professional sophist, and he is numbered among the sophists in Plato's dialogue that bears his name.
According to Aulus Gellius, the philosopher Democritus spotted Protagoras working as a porter, carrying a load of wood he had tied together with such geometric precision that Democritus concluded he was a mathematical prodigy. Democritus took him into his household and taught him philosophy.
In his lost work On the Gods, Protagoras wrote that he had no means of knowing whether the gods exist or not, citing the obscurity of the subject and the brevity of human life. This agnostic position reportedly led to him being expelled from Athens and his books being burned in the marketplace, though the classicist John Burnet doubted the reliability of that account.
Protagoras taught virtue, political life, the correct use of words, and argumentation. He claimed to teach the proper management of personal affairs, household management, and effective participation in public life. He also devised a taxonomy of speech acts and worked on the classification of grammatical gender, according to Aristotle.
Protagoras is believed to have died around 420 BC, after approximately forty years as a practising Sophist. Plato's Meno states he died at roughly the age of 70. His exact dates are not recorded and are extrapolated from surviving references in ancient texts.