It's checking to see if the $_GET query string variable "do" is set. If not, It's displaying a link to either delete the item, or return to the index. Notice in the href attribute of the link, "do" is set to 1. If you clicked on the link and it posted back to this page, the link choice would not display again, because do would be set, and its value would be 1.
In other words, the isset() function and comparison to 1 wrap the link, to only show it when needed.