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 also control the interval between the first and second followup.
There are 2 steps to turning on second followups.
Step 1: Enable second-followups by toggling the adjacent switch to blue within the campaign that you want to run second followups with.
Step 2: Choose the time interval between the first and second followup message. In the below example, all Leads that got this campaign's first followup 7 or more days ago, will receive the second-followup.