The dakuten are applied to the consonants to turn them into voice consonants. It's used to turn K-S-T-H into G-Z-D-B. The diacritic that turns H into P, and looks like a circle ゜, is called handakuten 半濁点, literally "half" dakuten.
Source: japanesewithanime.com (CC BY-SA 4.0)
Different Words
Two words with the same kana are different words if one has dakuten and the other does not. For example:- shinsha 新車
New car. - shinja 信者
Believer. - jinsha 仁者
Person of virtue. - jinja 神社
Shinto shrine.
Rendaku
Sometimes the first syllable of a word becomes voiced when it's suffixed to another word. This phenomenon is called rendaku 連濁. For example:- hito 人
Person. - koibito 恋人
Lover. Person who's loved.
Strange Tenten Usage
Besides its standard, normal use in turning consonants into voiced consonants, the dakuten can also e used make virtually any kana voiced.Notably, vu ヴ, a voiced u, isn't found in native Japanese words, only in loan-words, and, consequentially, it can only be typed in a computer in katakana, not in hiragana. (although you could type the dakuten separately: う゛)
Artistically, in manga, light novels, etc. the dakuten may be added to random kana when a character is screaming in order to denote added stress to the pronunciation. This is often done together with small kana.
Example: aaaahhhh ああああ゛あ゛ぁ゛ぁ゛
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