Multi-factor authentication is a way of increasing the security of your account. When you login to a service you are providing a “factor” of authentication, usually a password. This is referred to as “something you know”, but there are other factors such as “something you have” and “something you are”. MFA adds the ability for you to use “something you have” to also help prove that you are who you say you are, in this case a mobile device that only you have.
The way it works is that when you login to certain UniSA services you will be asked to provide your regular account and password, and then you may be asked for a code or to approve a notification that will be sent to your mobile device. This prevents a hostile party from using your account without your permission, as only you will have access to your mobile device so only you will be able to provide the code or approval.