Bracket/Parentheses:

It is a punctuation mark ()

to separate extra information or a comment from the rest of a sentence:

Mount Robson (12 792 feet) is the highest mountain in the Canadian Rockies. He thinks that modern musk (i.e., anything written after 1900) is rubbish.

to enclose cross-references:

This moral ambiguity is a feature of Shakespeare’s later works (See Chapter Eight)

around numbers or letters in text:

Our objectives are (1) to increase output, (2) to improve quality and (3) to maximize profits.