At its simplest, photo cropping is removing unwanted outer areas from the image frame. With more advanced cropping techniques, we can change the aspect ratio of the image from what the camera recorded, convert from horizontal to vertical or vice versa, and straighten horizons or other lines.
In digital photography, a bare bones, perhaps free, post-processing app or program is all we need for basic photo cropping. The tools for using cropping in a program are usually mouse clicks and dragging. The program may also have presets for constraining to a specific cropping ratio.