U+2017 Double Low Line

The Wikipedia has the following information about this codepoint:

An underscore, _, also called an underline, low line or low dash, is a line drawn under a segment of text. In proofreading, underscoring is a convention that says "set this text in italic type", traditionally used on manuscript or typescript as an instruction to the printer. Its use to add emphasis in modern documents is a deprecated practice. The underscore character, _, originally appeared on the typewriter and was primarily used to emphasise words as in the proofreader's convention. To produce an underscored word, the word was typed, the typewriter carriage was moved back to the beginning of the word, and the word was overtyped with the underscore character.

In modern usage, underscoring is achieved by markup or with combining characters. The original free-standing underscore character continues in use to create visual spacing within a sequence of characters, where a whitespace character is not permitted (e.g., in computer filenames, email addresses, and in Internet URLs). In contexts where no formatting is supported such as in instant messaging, or older email formats, the 'enclosing underscores' markup is sometimes used as a proxy for underlining the word(s) enclosed (_thus_).

In some languages, the mark is used as combining diacritic and is called a "combining low line".