Judges have stacks of submissions to read through and human nature dictates that they'll be looking for easy excuses to discard entries. Keep it simple and get to the point. Don't overwrite and make your compelling case on the first question.
Grab the judges' attention, keep them engaged, and tell them a story. If you're vying for a PR and Communications award, this should not be a tall order for you. Borrow some reliable techniques from journalism school and emphasize the 5 W's: Who, What, Where, When, and Why. (If there's space, maybe some How.)
Skip the jargon and get straight to the meat of things. Your judging panel will be comprised of some of the most senior and experienced professionals in the PR client, agency, and service worlds, but don't assume they know every in and out of your campaign or understand your specialized cant.
If you've gotten to where you are through behavioral interview questions, you should be familiar with the "STAR" method: Situation, Task, Action, Result. The video component is a great way to capture the heart of your submission and provide a good takeaway for the judges, but the story must be there for it to be memorable.