A series is emails a new subscriber gets on their own schedule: a welcome at once, something useful a day later, an offer in a week. It starts when the person subscribes or confirms.
Setting it up
Under the list's Welcome series, Add an email and set Hours to wait: for the first email, after sign-up (zero means at once, 24 a day later); for the next ones, after the previous email (24 is a day, 168 a week). Write each email as an issue.
Turn on starts the series. An unwritten email holds everyone at that step until it is written.
Pace
A person who signs up gets the first email right away. When many addresses are added at once, the emails go out gradually.
People who were on the list before the series was turned on do not get it. Imported people get it only if you tick Start the welcome series for those added.
Stopping
A person who unsubscribes leaves the series. The series stops itself when too many emails bounce or people complain, or when the sending server refuses email after email; you get a notification.
Only the last email of a series can be removed.