Default and standard variable tariffs are a basic tariff from an energy supplier. They are typically poor value and more expensive than a non-default, fixed-term tariff, which you can choose to switch to. You will often need to renew fixed-term tariffs after a year or more.
You could be put on a default tariff if your fixed-term contract ends and you’ve not shopped around to choose a new one, or if you've moved into a property and not agreed a new tariff with the current supplier.
Names of default tariffs vary, so if you aren’t sure if you are on one look at your last bill for your tariff details and then contact your supplier to check.