Without software, even the most powerful computer is just an expensive collection of copper and silicon, not even useful as a boat anchor. At the most elementary level, the computer CPU "understands" a set of operating instructions to do basic mathematical and logical operations like add, multiply, subtract and divide, and compare values, or move data from RAM into the CPU so it can be worked on.

If I wanted to multiply a value x by 2 then add a value b to that product, it might look like one of these instructions, depending on which brand or type of computer we're working on, or more specifically, which type of CPU. These instructions demonstrate two of the biggest issues that have and continue to face software development.

From a human perspective, it's not immediately obvious what these instructions are doing, not to mention a lot more typing compared to a purely mathematical expression. Secondly, there are different instructions depending on the CPU.

This presents two other problems. The same code can't run on multiple platforms or CPUs, and the developer needs to know the instructions for different CPUs. These problems aren't quite as bad as they initially appear, as software developers very rarely program directly against the CPU. We program against the operating system, so Linux, macOS, and Windows.

Programming against the operating system addresses the non-intuitive aspect of the CPU instructions. In the case of the mathematical expression example, no matter the operating system or programming language, coding the multiplication and addition will be very similar if not identical. 

However, the language is just hiding the platform and CPU differences from us. When we write code in a programming language, it still must be translated into the appropriate instruction set for the specific CPU. A programming language is comprised of two elements. The syntax or code we type and a piece of software, either a compiler or interpreter, that transforms the code into instructions the CPU can understand.

Many languages have different versions of their compilers for different operating systems and CPU combinations. Unfortunately, for all but the most simplistic programs, it's not a case of just running the same code through the appropriate compiler, called compiling the program, to get your code to run on any CPU or operating system.

There are usually some OS-specific elements of your code that prevent this. Earlier I said compiler or interpreter. A compiler turns your code into an executable program that can be run over and over independently of any other software. An interpreter is a software program that turns your code, known as a script in the context of interpreted languages, into CPU instructions as needed, on the fly as the script executes. This is often referred to as byte code or intermediate language.

Interpreted languages like Python cannot run without the interpreter installed. One side effect of interpreted languages is they execute more slowly as the CPU has to translate each language instruction into CPU instructions each time the program is run.