What is the meaning of ˘?

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

Source: wiktionary.org