What does document.domain = document.domain do?

http://stackoverflow.com/questions/1481251/what-does-document-domain-document-domain-do

I encounter a domain problem today and finally get it work by
document.domain = document.domain

Also, if you press the back button, the onload event of iframe will be faster than that of the parent frame and may cause permission denied error. The solution to this is to setTimeout and use the try-catch loop.