The term composition in photography describes the translation of 3 dimensional space into 2 dimensional image space. It includes all the elements, interactions, and boundaries that form a photograph. It's a decision of what elements to include and exclude.
Elements that complement the photograph and add to its purpose should be included. Elements that do not contribute should be eliminated.
Controllling the composition requires understanding of perspective, angles and lighting.
Good composition takes practice. Learn to walk around and look around. Ask what should be included and excluded. Look through your viewfinder. See how the image takes form. Try different angles, from above or below, or straight on. Try stepping back and zoom in to the subject, or try stepping closer and use a wide perspective.
Most amateur photographers feel the need to include everything in a photograph when most of the time just the opposite is true. Try to get close to the subject of importance before including other elements which could distract from it.
A photograph is not always portrayed as how we see with our eyes. We see in 3 dimensional space because we have two eyes. Therefore a scene that looks good to us may not look good in a photograph.
Take for example a trees when walking throught the woods. Now close one eye. When you close one eye the scene becomes less interesting. This is usually better observed using both eyes than in a photograph.
Now for example look at a mountain range. Distant mountains are more hazier than closer ones. Capturing that in a photograph helps tell our eyes more about the scene or three dimensional space. When our eyes look at the photograph, the hazier mountains tell us that they were more distant.
KEEP IT SIMPLE STUPID (KISS)
KISS applies to many things in life. It is important in photography. Simple photographs get the point across quickly without having the observer guess your intentions. First ask yourself why you intend to take a photo and why is it important. If its not important to you, why would it be important to someone else.
Get the point across quickly. Don't keep people guessing. People should not have to struggle to understand your photograph.