A segment is a subgroup of contacts on your list that you can send highly targeted messaging to. Segments can be created from an advanced search, when selecting a list during the campaign creation process, or from the Contacts > Lists Overview page.
Saved segments are accessed from the Contacts > Lists Overview page by clicking the down caret for any list, then clicking segments.
The List row will expand to show all segments that the selected list has permissions to edit:
Click on any segment to view/edit its conditions and view a list of contacts who meet the segment conditions.
Note that all segments will look for all contacts in your account who meet the segment conditions you set, regardless of which list they are associated with. The number of contacts for any segment will appear smaller when viewing the Campaign Summary page before sending an email.
To learn how to edit a segment, see this help article.
To learn how to create a new segment, see this help article.