This "horizontal bar" is called ダッシュ and serves more or less the same purpose as an em dash in English — inserting additional information to a sentence without ending the sentence or separating it completely from the sentence in parentheses (see what I did there?).

It can be used in pairs — like this —, or simply once.

Unicode does have

U+2E3A TWO-EM DASH ⸺

 

U+2E3B THREE-EM DASH ⸻

but I don't know if they are used by Japanese publishers, since they often have to typeset vertical text, as in your example. One also often sees "——" instead, especially on the internet. Depending on the font there will be small spaces, but everyone knows what is meant.

There is an obvious reason for using a longer dash (or several shorter ones): a single dash looks too similar to the 長音符

U+30FC KATAKANA-HIRAGANA PROLONGED SOUND MARK ー

This "horizontal bar" is called ダッシュ and serves more or less the same purpose as an em dash in English — inserting additional information to a sentence without ending the sentence or separating it completely from the sentence in parentheses (see what I did there?).

It can be used in pairs — like this —, or simply once.

Unicode does have

U+2E3A TWO-EM DASH ⸺

U+2E3B THREE-EM DASH ⸻

but I don't know if they are used by Japanese publishers, since they often have to typeset vertical text, as in your example. One also often sees "——" instead, especially on the internet. Depending on the font there will be small spaces, but everyone knows what is meant.

There is an obvious reason for using a longer dash (or several shorter ones): a single dash looks too similar to the 長音符

U+30FC KATAKANA-HIRAGANA PROLONGED SOUND MARK ー