Fundamental Idea of Programming
According to Hal Abelson in his foreword to the book Essentials of Programming Languages, the most fundamental idea in computer programming is this:
The interpreter for a computer language is just another program.
On the face of it, this seems not to apply to compiled languages, or programs written in machine code. But, upon reflection, it does. The interpreter for these languages is the microcode on the CPU, and the interpreter for the microcode is a logic program expressed in physical transistors.
In any case, the concept of an interpreter program is a powerful one, and is certainly worth study and mastery. As Sussman says, you will become "a designer of languages rather than only a user of languages, [ . . . ] a person who chooses the rules by which languages are put together, rather than only a follower of rules that other people have chosen."