The graphical user interface.
They don’t invent it (xerox PARC did), but Apple correctly identified that the user experience of existing computer systems was holding it back from being a thing everyone owns, and made computers a bad fit for many types of work that seem extremely obvious now (digital media creation particularly)
They did this more or less again with the smartphone: business folks and super nerds were the smartphone market before Apple. Now it’s the average person’s computer.