So this is a basic composition "Rule". I say rule but it's not really a rule, just some guidance. If you are stuck with the components of a picture then applying this guide can improve the composition - or so the folks in the know would say.
Here is a picture of the Lady's Tower in Elie in Fife. It's a reasonable example of this rule. Essentially you draw a grid in the viewfinder to create 2 vertical lines and 2 horizontal lines. You end up with 4 intersecting points and over one of those points you should position your main subject. Most cameras have grid that can be switched on and off in the view finder so that you don't have to guess.
Here's what the picture would look like with the grid switched on. You can see that the Tower is positioned over one of the intersections and some would say this makes for a better composition. These "rules" though, always come with the caveat that they are "meant to be broken". What you think looks best is really all that matters but keep an eye out for these tips and tricks as they can sometimes come in handy.