This is simply the Ionic dialect form of the future tense 'I will say,' equivalent to Attic ἐρῶ. Greek dialects preserved different forms of common irregular verbs, and ἐρέω represents the Ionic tradition found in Herodotus and other Ionian writers. The form itself is straightforward—a future tense form filling a gap in the irregular verb 'to say.' While Attic Greek standardized around ἐρῶ, Ionic maintained this older form. The various Greek dialects preserved alternative morphological traditions that grammarians carefully documented.