A word, phrase, or sentence, by way of comment or explanation, inserted in, or attached to, a sentence which would be grammatically complete without it. It is usually inclosed within curved lines (see def. 2 below), or dashes.
Parenthesis are used to control the order of searching in a database. If you use parenthesis around a phrase including an AND, OR, NOT statement that phrase will be searched for first before the other searching is done.