It indicates the previous hiragana is repeated. I don't see it much in everyday Japanese, but it's fairly common to see in lyrics for karaoke or more literary language. So, instead of writing ああ, for instance, you could write あゝ.
Thank you. I greatly appreaciate you!
Also worth knowing there is a voiced version of this, 「ゞ」 (see https://jisho.org/word/%E3%82%9E). That would change 「はゞ」 to 「はば」. Both of these are usually only used in people or company names these days, but used to be used more so you can expect to see it in older texts.
There is also the hiragana equivalent 「ヽ」 and 「ヾ」.
You can type all of these with the Microsoft Japanese IME by entering 「おなじ」and selecting from the suggestion list.
Slight typo: ヽ is for Katakana, and ゝ for Hiragana. And I guess you could call 々 the Kanji equivalent, which is seen very frequently in Japanese. In older vertical texts, 〲 is also common (unlike Hiragana GU ぐ drawn over the vertical space of two characters, not one) to denote repetition of the two characters that came before with voicing to spell words like for example 離ればなれ. Cannot really give an example here in horizontal text because it would look wrong.
Repitition mark
How is this used in language?