You might want to follow the link behind the first mention of the word "runtime": https://en.wikipedia.org/wiki/Runtime_system https://en.wikipedia.org/wiki/Run

You might want to follow the link behind the first mention of the word "runtime": https://en.wikipedia.org/wiki/Runtime_system https://en.wikipedia.org/wiki/Runtime_system So in a nutshell: "React as a UI runtime" considers React as implementing part of your UI's execution model. In other words: instead of your code having to call appendChild, setAttribute, etc. to modify the UI, React does that.