I often see "Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Pellentesque adipiscing nunc in purus venenatis facilisis. Integer a dolor. Phasellus ornare quam hendrerit augue rutrum porta" and stuff like that as filler on websites. What does it mean?

It's Lorem Ipsum filler and it doesn't tend to mean anything now, often being generated automatically. It's just used as dummy text that looks more natural when adjusting layouts than, say, repeating the word "text" 1000 times.

For more info: http://www.lipsum.com