This signifies the file was "dumped" (copied) from a physical retail cartridge or a digital eShop version into a playable format for computers.
Large games are often split into smaller "parts" to make them easier to upload and download. A .rar extension means it is a compressed archive. How to Use Split Archives ACNH [v0](Dumped).xci..part1.rar
Files from unofficial sources can carry risks. Always ensure your antivirus software is active when handling archives from the web. This signifies the file was "dumped" (copied) from