What it means; This is an environment and data structures that track everything that happens, how your program works. These are not only interpreted languages in which there is a runtime, but every compiled language. In C, the runtime environment is environment variables and services provided by the operating system that allow the program to interact with the rest of the system. In an object-oriented language, it is also all tables of objects and classes and methods that are created to allow the transmission of a message. In an interpreted language, this is the state of the translator, plus all these other things. In general, I think you could describe the runtime as "everything that happens when you don't explicitly write yourself."