The bare minimum is "I need to be able to send out a CV, file my taxes, and look up the weather". It used to be "configure DMA and IRQ channels so your sound card and hard drive don't conflict" but thank fuck we're past that stage.
I'm not teaching my parents the intermediate steps of installing Linux because none of those skills are valuable to them and what they use their computers for. I'll answer their questions if they ask, but normal people don't go into BIOSes, don't deal with ISO files, and these days I doubt most still use physical backups now that OneDrive and iCloud have been integrated so well.