This view is only available for files that are textual and it will display the contents of the file inside the tab, with the text color-coded to match the syntax of the programming language it is written in:
The syntax highlighting is based on the file type and file extension, so if you save a file with an incorrect extension (e.g. using .txt for a JavaScript file) it may not display the correct color-coding. For text files that are not for a specific programming language, it will simply display as plain text