Design is a very large field, with a long history and many diverse sub-fields and adjacent fields. When software developers talk about design, they are usually referring to one of two things; the first being software architecture design, which we talked about in Chapter 7, Building Tools. The other is the design of how people interact with software. The part of software that people interact with is usually called the user interface (UI).
As mentioned in the introduction, the idea of studying how users interact with the UI is called UX. In the SRE world, that often shows up as defining a command-line interface (CLI), but sometimes UX is designing a website interface. Other times, it is a graphical user interface (GUI), and sometimes it is a lever or button in real life.