I keep seeing this "separate presentation from content". What exactly does this mean???

Exactly what it says.

Content is the meat of your site, the message you're trying to convey.
Presentation is how you display it.

In this context, ALL of the styling, including the layout of the site, should be done
using CSS. CSS contains all the 'rules' for how your content is presented. No tables should be used for layout.

OK makes sense.. Thanks!