A programming language has two main aspects: (A) the permitted sequences of symbols making up a program, and (B) their meanings. (A) is often often called the syntax and (B) the semantics of the language. However, the notion of "semantics" or "meaning" in this context is often ambiguous as sometimes people who talk about the meaning of a program are referring to what objects it creates and manipulates in the computer and sometimes they are referring to things in the world that the program models or represents.
For example, if a program builds a database of information about rooms using a list of lists (like the example in Chapter 1), then a portion of the program has one meaning insofar as it specifies which lists are constructed or examined in the computer, and a totally different meaning insofar as the information manipulated is about rooms and their measurements, i.e. things that have nothing to do with the computer.