What does it mean that the single file upload only rigns at the time of post submission?

4. I don't understand what it means that the single file upload runs only at the time of the post submission. I guess I don't really grasp what happens when the user selects the SUBMIT button and how I am supposed to eventually see the result.

What he meant there is that when you attach a single file to your form it submits and the file is then uploaded to post with it afterwards.