For a number of these topics I'm going to point you to material that I think are some of the best free online resource, from excellent teachers. Mike Browne is one of those. An excellent photographer, with thoughtful explanations and examples. Firstly, take a look at his short video about aperture:
As you'll have seen on the Aperture video, as well as being one of the settings that effects your exposure (how bright your image is), your aperture setting also controls your depth of field. Depth of field is how much of your image is in focus, both in front of and behind the actual focus point. You'll have seen images such as portraits with a face clearly in focus but the background behind it blurred. That's a shallow depth of field, created using a wide aperture such as f/1.4, f/1.8 or f/2. A narrow aperture, such as f/18, puts much more of the image in focus.
A key technique in creating strong images is to 'identify the hero'. It may be obvious, such as in a portrait, what you want to be the subject of your photograph. But in a landscape or street image for example it may not be. Identifying the hero is deciding what the image is about, with the intention of drawing the eye of the viewer to that point in the image. And one method for directing the eye of a viewer is by using a shallow depth of field. Our vision is drawn to anything that is in sharp focus if most of an image is blurred.
(Our eyes are also drawn by other features such as the brightest point in an image, high contrast areas, recognizable shapes and diagonal lines.)