You can reduce the visible portion of the photo by cropping.
It is possible to crop freely or to do it while keeping a specific format or aspect ratio.
Aspects ratios
Typical aspect ratios are reflecting a sensor format, the most common one is 3/2, the one used with aps-c and full frame sensors, or also 4/3 for hybrid micro four third cameras.
Depending if your photo is about to be printed, you might want to choose another one.
To publish on some social media platform like Instagram, you could choose to crop with a square, to crop vertically a horizontal image or vice-versa.
When to use: sometimes you may find that only part of your image is more interesting and worth zooming onto. Or maybe the lens you used had a wide focal length and it was impossible to get closer to your subject without cropping.