What is the meaning of the "long hyphen," --, a DASH really, for grammar, translation, and punctuation? For example : シャワーヘッド (Shawaheddo)? Thank you
This 'long hyphen' is the elongation mark. It makes the vowel before it long. In English what we call 'long vowels' are qualitatively different from 'short vowels', but in Japanese the sole difference between long and short vowels is their actual length. Long vowels are held for about twice as long as short.
('er' sounds tend to become アー in translation, by the way)
In addition to Aeon Akechi's answer, notice too that the long-vowel mark is written as a horizontal stroke ー when the text is horizontal, and as a vertical stroke | when the text is vertical.
(Typography geeks: the vertical stroke above is the U+01C0 pipe character rather than a proper U+30FC long vowel mark displayed vertically, since I can't figure out how to get the Stack Overflow platform to show real vertically aligned text.)