A source file is an optional documentation string, followed by a sequence of semicolon-terminated declarations with optional trailing documentation strings, followed by an optional expression. A documentation string is any text between {{ and }} delimiters and may contain HTML.
Documentation strings are used in automatic HTML documentation generation. A documentation string that starts a file is used to describe that file in the module index, and it's also included at the start of that file's page. A documentation string after a declaration is used to describe it in the detail section of its file's page. The standard library documentation shows an example output.