checksum was given nothing to do!
By default, you get to this page when you launch checksum with nothing to do - no explorer context menu, no drag-and-drop, no command-line, nothing!
For full details about how to use checksum, see here. It is simple.
If you are not using the explorer context menu actions (or drag-and-drop), you need to provide command-line switches. Full details of the available command-line switches are here.
NOTE: You can customize what checksum does when launched with nothing to do, so instead of this page, you could have it perform some other task. All the details are inside you
checksum.ini (preferences file).
Have fun with checksum!