Trakt (https://trakt.tv/) is a website that “Automatically track what you’re watching. Find where to watch TV & movies and discover what’s hot.” Jellyfin will send information to your Trakt account about what you are watching.
I don’t use it anymore. Actually deleted my account a couple months ago. The purpose is supposed to make you be able to organize what you are watching, what you have watched and going to watch.
I can see its service 5 or 10 years ago but not really sure if it is necessary these days. I guess if you use multiple devices and media centers or services that can be linked to Trakt, it is ok.