A segment is a portion of your route (usually a few kilometers) that is timed.
There are 2 types of segments:
Private segments that you can create as you like from the application.
Official segments, supplied by BikeCompanion (there are now a few hundred of these, mostly corresponding to climbs linked to a peak).
During the recording of an ride, each time you come close to one of your segments or to an ascent with an official segment, a page is displayed on your smartphone with segment info in real time (distance, time compared to the best time, display of the “ghost” on the map, etc.).