Dynamic Content is anything that is not Static Content. If you have a script that pulls information in from a database and displays it on demand, that would be 'dynamic'.
I gave the example of a News Script. You might show a headline on the page that is pulled in from a database. Click on the headline, and the full story loads from the database and is displayed. Obviously the headline will take up less space than the full story. So any content below this must shift down to accommodate. Generally this doesn't happen in WWB. What you get is an article that overlaps, say, your page footer or any content that may be lower on the page. Or you get a box with a scroll bar.