Source code, in its purest form, is uncompiled program code.

In lay-mans terms, a programmer will write a website comprised of tags ‘<p></p>’ and the likes, which is then downloaded, and interpreted by your web browser. This, in effect, ‘compiles’ the code into what you, as a user, will see as the final product.

It is very similar to the process of creating a program that you may run within your Windows, OSX, or Linux operating system.

WP Glossary Term Usage