What is the meaning of (_)?

Encloses supplemental information.

Encloses a commentary.

Encloses optional variants or variant elements of words, inflections, spellings or pronunciations.

Encloses narration.

Encloses a letter or number starting an item in a list.

Encloses restored elements of text that were (intentionally) omitted by the original scribe as an abbreviation.

A transcription delimiter for silent utterances. It may enclose a transcription of a mouthed utterance derived from lip-reading, such as (ʃːː), or of the length of a pause between utterances, such as (1.3 sec). (...), (..) and (.) are used for increasingly brief pauses.

Used as quotation marks (e.g. in Arabic).

Encloses a mathematical phrase that has increased precedence in terms of operators.

Denotes the greatest common divisor of two integers.

Denotes a coordinate.

Defines a marked subexpression, to be matched using the backslash (\1, \2, etc.)

Used when defining a new function, enclosing the list of parameters.

Used when calling an existing function, enclosing the list of arguments.

Used with some expressions and conditionals, like "if", "switch", "for", "while", etc., in certain programming languages.

Encloses a repeated unit in a polymer.

Indicates that a number is negative in place of a minus sign.

sometimes used to enclose numbers to distinguish from letters

Encloses the pronunciation to a character the reader may not necessarily know.

Used to provide a deeper or second meaning to a phrase. Probably from Japanese ()(くん) (gikun, invented reading). Compare English read (used after a euphemism to introduce the intended, more blunt meaning of a term).

Added to the end of a message, with short text inside, as a sort of tone indicator.

Used to censor vulgar, profane or sensitive words or characters. Compare x in Latin alphabet. A formal way to censor words in Chinese is using × instead.

Added to the end of a message to indicate a lighthearted or joking tone.

Added to the end of a message, either left parenthesis only or both, used in place of ellipses …… (which can be read as indecisiveness) to imply an abrupt and intentional end to an incomplete sentence.

Used as a plaintext fallback of ruby: encloses a regular or traditional pronunciation to kanji (sometimes other types of words, e.g. katakana in parentheses following an English word) that the readers may not necessarily know how to read.

Used as a plaintext fallback of ruby: encloses an invented reading or a pronunciation borrowed from another language, which follows usually but not necessarily a kanji word, to conveys the word's deeper or second meaning (see ()(くん) (gikun)).

Used to censor vulgar, profane or sensitive words or characters. A formal way to censor words in Japanese is using × or instead. Compare 自主規制 in Japanese and x in Latin alphabet.

Added to the end of a message, with short text inside (the right parenthesis is optional), as a sort of tone indicator.

Added to the end of a message to indicate a lighthearted or joking tone (the right parenthesis is optional),.

sometimes used to enclose digits to distinguish from letters

Source: wiktionary.org