It is a very confusing word. It has many different meanings. One of them is "to separate something," "to mark it as being apart from something." When talking about punctuation, punctuation marks can be used to indicate that something is a little bit separate from the rest of the sentences. We can say that the marks are used to "offset" the phrase. "All of the students, except for Mary, put down their pencils." "All of the students—except for Mary—put down their pencils." "All of the students (except for Mary) put down their pencils." In the first sentence, the phrase "except for Mary" is offset with commas; in the second, with dashes; and in the third, with parentheses.