U+FF70 Halfwidth Katakana-Hiragana Prolonged Sound Mark

The Wikipedia has the following information about this codepoint:

The chōonpu (Japanese: 長音符, lit. "long sound symbol"), also known as chōonkigō (長音記号), onbiki (音引き), bōbiki (棒引き), or Katakana-Hiragana Prolonged Sound Mark by the Unicode Consortium, is a Japanese symbol that indicates a chōon, or a long vowel of two morae in length. Its form is a horizontal or vertical line in the center of the text with the width of one kanji or kana character. It is written horizontally in horizontal text and vertically in vertical text (ー). The chōonpu is usually used to indicate a long vowel sound in katakana writing, rarely in hiragana writing, and never in romanized Japanese. The chōonpu is a distinct mark from the dash, and in most Japanese typefaces it can easily be distinguished. In horizontal writing it is similar in appearance to, but should not be confused with, the kanji character 一 ("one").

The symbol is sometimes used with hiragana, for example in the signs of ramen restaurants, which are normally written らーめん in hiragana. Usually, however, hiragana does not use the chōonpu but another vowel kana to express this sound.

The following table shows the usual hiragana equivalents used to form a long vowel, using the ka-gyō (the ka, ki, ku, ke, ko sequence) as an example.

Onbiki may also be found after kanji as indication of phonetic, rather than phonemic, length of a vowel (as in "キョン君、電話ー").

When rendering English words into katakana, the chōonpu is often used to represent a syllable-final sequence of a vowel letter + r, which in English generally represents a long vowel if the syllable is stressed and a schwa if unstressed (in non-rhotic dialects such as Received Pronunciation; in rhotic dialects (such as General American) it may additionally be an R-colored vowel). For example, "or" is usually represented by a long ō (oo or ou) vowel, with the word "torch" becoming トーチ tōchi.

In addition to Japanese, chōonpu are also used in Okinawan writing systems to indicate two morae. The Sakhalin dialect of Ainu also uses chōonpu in its katakana writing for long vowels.