Well, to begin, there are what is known as "interpreted" languages and then there are "Compiled" languages. Most scripting engines, like PHP or CSS use what is known as an "interpreted language." C, C#, C++ and other languages that the end user never really sees the source code for are called "compiled" languages. There are languages such as Python which could be either, depending on how you write and use Python. Basically, when a developer writes code they write it in a language they can understand. The programming language itself has definitions for what all of that code means, and when the computer Interprets code it basically just reads one line at a time and breaks the code into "machine code" that is, quite literally, the literal translation of what the developer wrote to something that activates the correct buses and instructions inside the CPU. Interpreting code means that the variables in the code are replaced with their corresponding values "just-in-time" before the interpreter reads them. That means that a simple script containing code is read and converted to machine code each time it is executed.