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. Wikipedia

Diacritic mark in the Greek alphabet Wikidata

A small iota written below an alpha, eta or omega – e.g. in ᾠδή (ōidḗ) – representing a vocalic offglide that was pronounced in early Ancient Greek, but later became silent; invented in the Middle Ages in order to mark those vowels which would originally have had such an offglide. Wiktionary

Small iota written below a vowel letter. Wiktionary (translation)