Don’t worry; it won’t affect your files, data, apps, user settings, etc. Only a fresh copy of macOS High Sierra will be installed on your Mac again. … A clean install will delete everything associated with your profile, all your files, and documents, while the reinstall will not.

Does installing new Mac OS delete everything?

Reinstalling Mac OSX by booting into the Rescue drive partition (hold Cmd-R at boot) and selecting “Reinstall Mac OS” does not delete anything. It overwrites all system files in-place, but retains all your files and most preferences.

Do I need to keep Install macOS High Sierra?

The system does not require it. You can delete it, just keep in mind that if you ever want to install Sierra again, you will need to download it again.