Line of sight is a pretty simple concept for photographers.
It’s the invisible line between a subject’s eyes and the object at which they are looking. It is implied. It is invisible.
It’s also very powerful to a photographer.
How so?
Well, it can be used to draw the eye of the viewer to wherever you want. The main center of interest for example. It stops the viewer’s eye drifting off the image.
Perhaps a few examples to demonstrate this in action will show you how powerful this can be.

This photo uses the line of sight to draw your eye from the woman to the edge of frame.

This photo takes your eye from the man to the bird.
As you can see, it is a useful tool in the composition arsenal of a photographer and it’s one you should absolutely experiment with.
It’s also one of the techniques on Kent Dufault’s Action Cards where he goes into detail about how to apply it in other scenarios and gives you several assignments that you can do today to practice this technique. Take a look at them here.