"You're selecting everything within the box. It's used for copying and pasting, as well as printing.
You'll find that if you now press Ctrl + C (or use the Copy entry on the Edit menu) and then paste into another application, then everything within the border will be copied.
Similarly if you go to the File -> Print and then click the Selection box, only the part of the page that you've selected will print out. This is a great way of saving paper when you need to print some information from a page that's covered in adverts, etc. that you'd rather not print out."