A question about the root address

I am now trying to figure out “api” apparently its part of sonarr and mentioned A LOT by the sonarr log. I went to Servarr Wiki file and searched for “api” and it referred sonarr and sonarr trouble shooter searched there and then went to sonarr settings. Basically there was no “api” info anyplace. I must be missing something here.

Then there is something called “Arr”. Have no idea about that one.