If I’m understanding the code you posted correctly, it does the following:
when you hover over a anchor link in the a list item tag (which will be Home, Forums Videos, Downloads and Snippets), the width of that element will be 24%. Also, since you’re hovering over an li element that is inside a ul element, the active tab of the navigation (defined by the .active class) will be shrink to 19%. However, should you hover over the active tab, you don’t want it to shrink to 19% and thats what the last line does; it overrides the line above that, so when you hover over the active tab, it will stay at 24%.
I hope that makes sense.