As many have pointed out, this is more of a guideline than a rule. But applying "The Rule of Thirds" can help you achieve better shot composition.
Essentially you divide the frame into thirds horizontally and vertically. Then you attempt to place important elements of your composition where those lines intersect.
Here's an article from Videomaker Magazine which discusses how the rule applies to both 4:3 and 16:9 screen aspect ratios.