Documentation is a description of code written in day to day language. Questions using this tag can ask about teaching how to write proper documentation.
Documentation of code is a description of how the code works and/or what it does. It is considered a good coding practice to document your code, because it makes reading it clearer and simpler.
Documentation should be making the operation of a program easy to understand without going too deep into details. There are many tools to create a document from a project. Some languages, such as java, have tools supplied by the developers to create documentation.