What is composition?
Composition in photography is the act of thoughtfully arranging or ‘composing’ an image - whether through arrangement, framing, angle and more - to express an idea or concept.
A Little More Detail:
Photography is just one of the many forms of art based heavily on composition, and it’s both quite a specific and conceptual expression. Specific, in the way that there are universal styles of composition - such as the rule of thirds and the golden ratio - and technical ways to compose a photo, including framing, arrangement, symmetry and straight lines; and conceptual, in that the idea of composing a shot is to think how your photo can best convey what you’re trying to say through an image. It’s putting a little more care and thought into what exactly you’re shooting and why.
For example, if you’re shooting a tall building, do you want to capture its enveloping size? If so, you might think about shooting from a below looking up. Do you want to include some shadow given off by the building? Maybe you need to change angles. Is there anything you can include within your frame - people, trees, cars - to further emphasise its scale? Composing a photograph before you shoot allows you to tell your story better through images.