http://www.google.com/search?hl=en&q=debug+edit+and+continue&btnG=Google+Search&meta=
The first link has a nice example of how you can use this feature:
http://msdn.microsoft.com/vstudio/tour/vs2005_guided_tour/VS2005pro/Smart_Client/EditContinue.htm
I like this feature, say you are miles deep in code and you realise that you have spelt a variable name wrong:
$name = $nam + ' hello '; // should be $name = $name . ' hello ';
As you step onto this line, you edit and fix it, and save and upload, then continue debuging.
Cheers,
John