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