Negative margins are typically used to reposition objects on a page. It's difficult to be more precise without knowing the context and the HTML being affected. Sometimes, they can be misused.
Looking at the source code for the page that you referenced, I found the following:
margin: -10px -10px 0;
Because this is the first container inside of the <body> element, I wondered why the negative margins were being used.
The reason is simple enough...
Browsers apply a default margin (assumed to be 10px) inside the viewport. This author assigned the negative margins so the background image in that first container (when it's visible) would flow to the very edges of the viewport.
Negative margins can be amazingly useful but are often misapplied because the coder is unaware of HTML defaults, such as those around lists.
This is a pretty simple answer. Hope it helps.