Jackett is a program that allows you to configure multiple indexers (torrent sites, like 1337x, EZTV, RuTor, Nyaa.si, etc.) in a single interface, that way you can search through all of them at the same time. Jackett, and another program just like it called Prowlarr, is usually used in conjunction with the .arr suite of programs (Radarr, Sonarr, etc.), but it includes a manual search function that allows you to query all the indexers you have set up in the interface at the same time. That's exclusively what I use it for.

So, for example, I have 22 indexers set up in my installation of Jackett. I can use the manual search function to search through all of them at once, then I can sort the results by seeder count, publish date, and file size, and I can filter through the results to find exactly what I'm looking for. Once I've found the file I want, I can copy the magnet link directly from the search results and paste it into Qbittorrent. It's an extremely easy way to find files quickly, and it's much more efficient than manually going to a bunch of different torrent sites to search for a file that might not even be available there. With Jackett, I've literally never once had a case where I wasn't able to find what I was looking for. That's how good it is.