What does this do?

[prezet]

Hi guys

I was looking at some css code yesterday for a rollovers and came across this page:
CSS Rollovers (http://www.webcredible.co.uk/user-friendly-resources/css/rollover-buttons.shtml)

I understand what the code is doing all apart from this one piece:

* html a:hover
{
visibility:visible
}

The code definitely needs it to work, esp in IE, but I'm not really sure what it's doing?? Any insight would be great.

Thanks