What is the meaning of ˘?

Used to mark a short/unstressed syllable in a metrical foot or verse: e.g. ⟨˘ˉ⟩ iamb, ⟨ˉ˘⟩ trochee, ⟨˘˘⟩ pyrrhic. Also placed over vowel letters: see ⟨◌̆⟩.

Source: wiktionary.org