The Second Time Opt-In Message is sent to subscribers that join your list, after previously joining and unsubscribing that list. You can learn how to set this message in the post on how to set the Opt-In Message, as it's located in the same place.
An example of a common Second Time Opt-In Message is "Welcome back to our mobile VIP club", where a common Opt-In Message is "Welcome to our mobile VIP club".