A segment allows you to work with a select portion of your subscribers on a mailing list as opposed to a full list.
You can easily create a new segment - or edit an existing segment - right from the left-hand menu by choosing Subscribers > Segments:
As always, if you have any questions, please don't hesitate to let us know. You can find all of our great support resources and how to get in touch with us via email, chat, or phone on our Support Page.