 Here's the general idea. Say a survey finds 52% favor candidate red and the error margin is plus or minus three percentage points. That means there's some chance as many as 55% back red, 52 plus three. It could also be as few as 49, 52 minus three. Of course, the error margin also applies to the portion supporting candidate blue. In this example, the difference between red and blue would need to be more than six points.