Default Reply can be set in Automation>Default Reply tab.
It is sent when your subscribers type in something your bot cannot recognize, for example — they typed in something that doesn't match any of your Keywords.
It is a good call to provide some contact information so your subscribers can reach out to admins instead of asking the questions your bot cannot answer. You can also use Buttons and Quick Replies to forward them in the right direction in case they get lost or even start a long Flow inside the Default Reply.
In case your Default Reply is enabled (you can turn it off with the toggle on the top) select one of two options: send it once in 24 hours or send it every time it is triggered by unknown inputs.
We also added an ability to capture your subscribers' inputs even before Default Reply goes off. You can use Last Text Input Custom Field that stores the very last input and can be sent to Admins as a notification or to external servers that will return dynamic content based on their input.
For example, your Default Reply can contain only one Action Step with Notify Admins Action that will immediately notify you about the question that was asked in a conversation.