Site Url & Absolute path

Site URL:
This feature basically sets the Domain Setting.

Its usually filled in with the installation, if another domain is set, your downloads will be pointing to the location of another domain.

Absolute path to site:
This is the full path to where you script is run from, changing this values alters the location of the script. This factor is also set up with the installation. So in most cases its not needed to change.