to see if the device is connected to the internet. If it doesn't get the success response it assumes you are behind a captive portal and pops a webkit window so you can do the portal dance. This is mostly useful if you are using thick-client apps, since if you're using a browser you're going to see the portal page as soon as you go anywhere.
To disable it, set this preference:
sudo defaults write /Library/Preferences/SystemConfiguration/com.apple.captive.control Active -boolean false