Not everyone takes photos with the horizona perfectly horizontal.  The “Straighten Tool” can be helpful to quickly rotate the photo so that the horizon is flat.

“Cropping” a photo means that you can cut out parts of the photo that are not wanted.  Suppose you have a person(s) off to the right of your central focus person(s).  You could crop out those unwanted persons.  Of course, realize that the image size, could be reduced some, and you may develop pixelation when the image is zoomed in on.