The iota subscript is a diacritic mark in the Greek alphabet shaped like a small vertical stroke or miniature iota ⟨ι⟩ placed below the letter. It can occur with the vowel letters heta ⟨η⟩, omega ⟨ω⟩, and alpha ⟨α⟩.
Iota subscripts appear in the diacritics used in New Testament Greek text. Restoring the iotas they represent affects the numeric structure and design of the Bible.
Wikipedia has all you may want to know about iota subscripts.