the "Collections" view

[TheDreadPirate]

Jellyfin pulls down metadata from (I think) TMDB, which includes whether movies are part of a "collection". If two movies in your library are part of the same "collection", as defined by TMDB, they will be put into a collection together. This is usually for movies that are part of the same series. Like all of the John Wick movies, they would be put together in a collection. Or the Evangelion Rebuilds.

If you've already scanned your library before enabling that setting, you should be able to just "Scan all libraries" from the dashboard home. It should create the collections.

You should also consider installing the TMDB Box set plugin. This will improve this process, including pulling down collection images.