Parenthesis are a pair of brackets used to mark a phrase in a sentence. They contain important details but do not affect the grammatical structure of the remaining sentence.
They also help the reader maintain focus on the rest of the sentences, while also providing extra information. Interestingly, they weren’t used often in the 1800s and they began to be used more widely in the last half of the 20th century. They are used with greater frequency in math sentences and have a distinct purpose in that context. The word comes from Greek and literally means to put in beside.