I was curious about this too, and looked into it. It's referring to a suite of piracy apps, for automatically building libraries from trackers.

Wiki (linked from one of the githubs, has links to all apps and more info than the githubs): https://wiki.servarr.com/

Lidarr (Music): https://github.com/Lidarr/Lidarr

Radarr (Movies): https://github.com/radarr/radarr

Readarr (Books): https://github.com/readarr/readarr

Sonarr (TV): https://github.com/sonarr/sonarr