"Segments" are used to be able to segment your audience and use the segments inside of your "Flows" or "Campaigns" to send out personalized content to specific user cohorts.

You can click on the "CSV" Icon to export a specific segment as a CSV file.
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If you hover over the eye icon you can see what the specific segment is using as filters to include/exclude specific users.

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Alternatively here is a video explaining the "Segments" feature:
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