One of a pair of symbols or together, [ ], they are used in writing or mathematics for showing that the piece of information or set of numbers between them can be considered separately.

[Brackets]

Brackets are like a thin man and twin brother
Head to head,
Toe to toe,
Standing facing each other
Though much like parentheses,
Brackets are squarer.
They're used when an editor
Edits an error.
For instance, "The mesengger [messenger] ran
To get some [word missing]
To hel[p] the poor man,"
And also for putting in facts; such as, "He
[George Washington]
Chopped down a [cherry] tree."

You'll learn, if you type,
And it's bound to displease,
That brackets aren't found among typewriter keys.
You leave a small space
For each bracket and then
Go back and make brackets
With pencil or pen.
If you are a typst, thus treated unfairly,
Be grateful that brackets are used very rarely.
[Fortunately, these days, this typing problem does not exist with a computer keyboard].

The brackets unit of punctuation marks.