Parentheses are punctuation marks that we use to set off material that could be omitted without changing the meaning of our sentence.
- That man over there is my uncle (the one with big moustache under his nose).
- He had to retake his exam. (He had failed it the first time.)
Remember that "parentheses" is an irregular plural of "parenthesis". That's why we say: "there is one parenthesis", but "there are two parentheses"!