Segments are saved, reusable audiences: a named set of tag and metadata filters (a filter group) that you can apply as the audience of an email instead of rebuilding the same rules every send. Create, rename, and delete them from the audience picker while composing an email, or via the API.
Note that an email's audience is a snapshot: applying a segment copies its filters onto the email, so editing or deleting a segment never changes emails you've already sent or scheduled.