First, What is User Input Flow? With user input flow you can ask a single or series of questions from users inside Messenger. Once users give any answer, then the next question will be sent if there are any. These answers will be saved in the database & also can be saved as custom fields. Custom fields can be used as variables in Messenger reply.
Next, What is Variable? After you have saved a response in Custom Field, you can use it as a variable in your message reply to the subscriber.
The last one is How to use a Variable? To use the variable for Custom Field, write the variable surrounding by # like (#Custom Field#).