When you crop an image or photo, you remove part of the image. This may involve cutting some of the image from the left, right, top, bottom, or any combination thereof. It may also mean just cutting out a small rectangular of the image.
For example, your friend takes a picture of your family and leaves a good five feet of headroom above the tallest person. If you are going to use the photo as your Christmas card picture, you probably don't want half the picture to be filled with sky. Therefore, you can use an image-editing program to crop the top part of the image, leaving only a small area above the tallest person's head. If the camera was zoomed out too far, you may want to crop some of the sides as well.
Most image editing programs include a crop function. Typically, you select a rectangular area using the mouse and click "Crop" to remove the area outside the selection. Once the image is cropped, the cropped section is discarded, leaving only the remainder. Therefore, if you are working with an original photo, you may want to create a duplicate file before cropping and saving it. Note that cropping is different than simply reducing a image's size, which involves lowering the image resolution.