Extras are, as the name suggests, add-ons you can buy to add extra value to your Mobile plan or Prepaid phone - there are Talk, Text and Data Extras, as well as bundles that combine them. Most Extras are monthly and some are one-off, so you will be able to make sure you have what you need when you need it. Simple.
If you use up the included amount in an Extra, you'll simply go back to the standard casual rates. You can cancel Extras at any time.