What is the meaning of ¯?

a mid floating tone.

a level tone.

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

high rising tone.

The first tone of Mandarin Chinese in Zhuyin script.

Source: wiktionary.org