What is a parenthesis in ordinary English sentence usage?
In ordinary English sentence usage, a parenthesis is an inserted word or phrase placed in the middle of a sentence to enlighten or describe it, set off by little curves. It interrupts a sentence that already makes sense, adding explanatory material before the sentence continues.
Les explains that in ordinary English, you can have a sentence that makes sense, and then you think of something you can stick in the middle that would enlighten it and maybe describe it a little bit. You open your sentence up, put the parenthesis in the middle, and put the little curves around it. He uses this to illustrate Romans 9, 10, and 11 as parenthetical chapters, where Paul suddenly digresses to deal with the Jew—Israel's past, present, and future—before returning to where he left off in Romans 8 and going into chapter 12.