He had added a Jellyfin repo to his apt sources.list file for some reason, which is weird and likely not the right way to do it these days. But it might have been in the past, so it could be OP was following some obsolete procedure (or one AI-hallucinated from an obsolete procedure).
After realizing that OP was completely going about it the wrong way, the guy you replied to just looked up the correct way and relayed that to him.
See also: https://wiki.debian.org/DontBreakDebian