'SMS Fallback' is a feature where you can choose to send an SMS to your users / parents if they haven't read your alert within a certain amount of time.
Let's say, for example, that you need to close the school early at short notice and need to get a message to the parents. This is urgent, and you want to make sure you get the message to everyone. So set a timeframe, eg. 30 minutes, and if the user hasn't read the alert within that timeframe, an SMS will be sent to their phone and an email will also be sent to their email address.
This also goes for users who may not have downloaded your app. If it's urgent, you know the SMS and email will be delivered.
This feature is included with the basic School App, but any SMS cent will be charged separately at 6c per SMS (including VAT).