Apple Mail already allows to send an email to a group, i.e., multiple people. If you do so, everyone will receive the same email, of course.
With Serial Mail you can create an email template containing certain placeholders (e.g., for the name of the recipient) which are then replaced by the corresponding value from the recipients address book before sending the email.
Serial Mail does not replace Apple Mail. It does not send the emails. Instead each generated email is showed for review and, if accepted, placed in your Apple Mail's outbox. In other words, it takes your template, merges it with your Address Book data and puts the generated email in your Apple Mail outbox.