The Measure of all things?
This quotation is restated in Plato's Theaetetus at 152a. Sextus Empiricus gives a direct quotation inAdv. math.7.60: πάντων χρημάτων μέτρον ἐστὶν ἄνθρωπος, τῶν μὲν ὄντων ὡς ἔστιν, τῶν δὲ οὐκ ὄντων ὡς οὐκ ἔστιν. ... Similarly, in Ancient Greek, Protagoras used the Greek word anthrōpos (human being, person), making a general statement about human beings.
What I think the phrase means is that it reveals that we understand the world through the lens of our own perspective. The world is experienced on a non-verbal direct sensory level. However, once we begin to process that non-verbal data we select and transform it into our human symbols systems. Hence we are the measuring tool that forces all things to conform to. We only understand the world through this process of selection and interpretation.