Is Mac a computer or an operating system?
Mac is the brand of computers (specifically, the hardware) made by Apple. The Macs on sale today are the MacBook, MacBook Pro, MacBook Air, iMac, Mac Pro, and Mac Mini.

Mac computers run on macOS (which was called OS X up until its rebranding in mid-2016). macOS, not Mac, is the equivalent of Windows, but since Apple wouldn't ask "Are you a macOS user" (the equivalent of "Windows user") and wouldn't ask "Are you a Windows computer user" (the equivalent of "Mac user"), they use the fairly reasonable grammar found in your screenshot.

You can also use an anology to sum it up: Mac is to PC, as macOS (previously OS X) is to Windows.

TL;DR: The first line of Wikipedia's entry on macOS pretty much sums it up:  


  macOS is a series of graphical user interface–based operating systems developed by Apple Inc. for their Mac line of computer systems.