"Metadata" entered the lexicon thanks to the Edward Snowden revelations, prompting explainers on what the heck it is. The prefix "meta" is self-referential; metadata is data about data. To put it in a less confusing way, metadata is extraneous information about data.
You've got a document, and the content of the document -- the words -- are the data. But the document also contains other information about the data, like who authored the document and when it was created, and what parts of the text are underlined. This hidden data can present an entirely separate set of ethical problems.