Database driven websites are websites in which selected information on pages is pulled in from a database. Every time the page loads, the appropriate information on the web page is pulled in from the database.
A good example of a database driven website is an online banking site. Every time you check your balance online, the figure is pulled from a database. If the data had to be updated manually by a content editor on the website ever time someone withdrew money from their bank account, the system would be impossible.