Segment is a customer data platform (CDP) that helps you collect, clean, and control your customer data. Here’s more information [1] from Peter Reinhardt, one of the co-founders.
It's basically a really complex distributed pub/sub tool. It ingests data from sources, say Shopify and Heap Analytics, and can then send that data to 1 or more other supported platforms along with some user-defined transformations. Like a really fancy IFTT.
[1] https://www.quora.com/What-is-Segment-io-all-about