Not OP but: they are apps to schedule automatic downloads. Like Star Trek Strange New Worlds but don't wanna go every week and download it manually? Set it up in Sonarr and when you wake up the new episodes are waiting for you. Radarr is the same for film, prowlarr for aggregating torrent trackers all in 1 spot, bazarr for subtitles, and there's a few others. Can also be set up with usenet in addition to torrents.