Seam Carving GUI is a program to help you resize an image without affecting its main subjects: buildings, people, animals or whatever it might be. Instead the program aims to remove only content which isn’t visually important, a technique known as Content Aware Image Resizing (CAIR). The program works by first allowing you to mark either the areas you’d like to keep, the areas to remove, or both. It’s a straightforward, if slightly tedious process: you just “paint” over that area with a brush.
If you want to resize the image to specific dimensions then you can optionally enter them here. But if you don’t really care, simply click “Remove” and Seam Carving GUI will remove the areas you’ve specified and show you what’s left.