Segments are the subsets of your user base. These are the group of users bucketed based on the criteria of interest you have defined by analyzing the events and user properties such as their demographics, actions, spending pattern, behavior and so on.
To quote an example, a segment may be the app users who made a purchase in the past 15 days or the female users who visited your website and purchased more than 2 products.
Segments serve as a specific target market which enables you to understand your users better, design and drive the strategic marketing campaigns leading to more efficient engagements and conversions. They also help you to capture your user’s response to your marketing campaigns, significantly helping you to make educated marketing decisions.