Density - the degree of compactness of a substance in an outlined area.
Some websites have a lot to say; and they should be able to say all of the things they would like to say. When websites say too much, the page becomes too dense. When a website become too dense with text or graphics, the messaging can be too overwhelming for the viewer. Finding the balance of text, graphics, imagery, and blank space is tricky. That is why most businesses hire someone like me to create their website.
An easy rule to page density, is to create a small amount of copy which is rich in content. Catching the viewer's attention with a short "want to learn more" tone leads people to continue reading and learn about all which your business has to offer.