"!important" <- What is That?

[kapaha]

I was browsing a site for something to do with compliant flash and all that, and after becoming bored and moving on to some other thing I wasn't really reading, I came across this:


.sIFR-hasFlash h1
{
visibility: hidden;
}
.sIFR-flash { visibility: visible !important; }
.sIFR-replaced { visibility: visible !important; }
span.sIFR-alternate
{
//and so on...


My issue is: What on earth is that "!important" thing? What does it do?
I've tried looking everywhere for this and I can't find it.

Thank you.

[_Aerospace_Eng_]

http://www.w3.org/TR/REC-CSS2/cascade.html#important-rules

[kapaha]

Thank you very much. I'm glad there's someone else to navigate that awful w3c site, because I can't spend more than thirty seconds there without leaving in disgust. :D

Thanks again.