Plain text is loosely used to describe data that consists only of characters (as opposed to images, binary data, etc). The reason it is so plain is because it does not contain any styling.
One of the main advatanges of plain text is that it separates the file format from the editor, in that multiple editors can be used for the same file. This is particularly useful for code, ensuring it is portable to different machines.
Additionally, storing knowledge as plain text ensures that it will be recoverable in the future. Knowledge stored in a proprietary rich text format or as binary is not always recoverable, as it requires a key, namely the means of decoding the formatted data.
This is an example of Plain Text