The Opening Brief is your explanation to the Appellate Court of why the trial court was wrong. After the transcript on appeal has been sent to the Appellate Court, you get a Briefing Schedule, that gives you a deadline by which to file that Opening Brief. Unlike the trial court, the Appellate court is must more flexible, so if you need more time, you can just ask for it, explain why, and they usually give you the extra time you request, like 30 days.

You may need this extension because the clerk's transcript is incomplete, or you didn't get it at all, or just didn't have the money yet to pay for the opening brief to be written. The request needs to be filed by the deadline to avoid a default, but otherwise it is simply enough to do.