“An unattended login is a feature that offers the ability for the end user's data to be synchronized without the end user having to log in every time.
The end user needs to log in every 90 days for security purposes and to verify they are still actively using the given service.”
When you log in the first time, you are required to give your credentials and follow a login process similar to what you do when logging in to your bank. This process is referred to as a supervised login.
In most cases you do not need to go through the process every time. This is referred to as an unattended login.
For security purposes, you need to log in again (perform a supervised login) every 90 days.
Unfortunately, there are still some banks that don't support this feature. If you're a customer of one of these banks, then you'll have to use your username and password every time you want to update your accounts and transactions.