When you're doing something you have a lot of factors that contribute to you reaching your goal. These factors add to the success of your goal being achieved. When one says they have the "element of surprise" that means that they have surprising their intended audience as one of these factors. It means that their particular audience won't see what's coming next, so it makes it easier for one to reach a goal.
Such as....a magician has the element of surprise to make a magic trick more exciting because the audience doesn't expect it.
Hope it helps. Good luck!