RAR is a compressed file format (like ZIP, but often with higher compression). To split a large macOS installer (approx. 5GB) into a smaller download, repackers use RAR.
If you are using this file for a virtual machine or a bootable drive, you might find these GEEKrar guides helpful: macos high sierra dmg by geekrar.rar password
macOS High Sierra is a major update to Apple's operating system, released in 2017. It's known for introducing a new file system (APFS), improvements to Siri, and various under-the-hood changes. RAR is a compressed file format (like ZIP,
A step-by-step guide to installing macOS High Sierra on VirtualBox . improvements to Siri