Segment is a tracker that allows you to pipe the data to any tool that supports segment. That way you can change or add tools without touching the tracking code in your app or service.

Segment is a product that allows you to write events once and then allows you to send them to whatever tools you want.

If you use amplitude or Mixpanel you're locked in with an SDK and it's harder to switch vendors. With segment you can write events once and decide afterwards where to send them.

This is useful as your company grows and you start using analytics events in BI tools, Marketing tools (like Intercom) and product analytics tools (like Amplitude)