If the Lead didn't respond after the invite message, or after the first followup message, then they will get the second-followup message.
You can control the interval between the first and second followup.
Choose the time interval between the first and second followup message. In the example below, all leads that got this campaign's first followup 7 or more days ago, will receive the second-followup.
If the 'Exclude Responders from Followups' button is not enabled, it will still send the second follow-up message as well as the first.