Segments are parts of rides, and can be compared against each other here.
Segments can be created for individual rides using the tools on the ride page. Alternatively, segments for intervals recorded with a power meter can be automatically detected after some rules are set up on the automatic segments page.
Segments are compared against all other segments with the same name, and the summary data can be shown for either individual segments or with the segments in each ride being averaged together.
The numbers in the list of segment names indicate how many rides contain that type of segment, and how many segments there are with that name. Selected segments can be deleted or renamed.