If you have a job interview, after the interview, you send him/her an email, the contents can be anything, mostly a " thank you" note. this email is a follow up, because the email was sent after the Interview. An email that was sent to follow up the previous act is a follow up email.

Hello Follow-up email means sending an additional email to remove confusion. For example you might send an "original" email saying: "Hi, I want to meet you on the weekend" Your friend sends an emailed reply: "Sure, when do you want to meet?" Then you send a followup email saying: "I want to meet you this Saturday at 10am" The followup email removes any confusion or misunderstanding about what you want. Does that answer your question?

It just means another email replying to the one you sent someone.