http://uwb25d43nnzerbozmtviwn7unn7ku226tpsjyhy5n4st5cf3d4mtflqd.onion/posts/metadata/index.html
In the context of files, this can mean information that is automatically embedded in the file, and this information can be used to deanonymize you. For example, an image file will often have metadata about when it was taken, where it was taken, what camera it was taken with, etc. A PDF file may have information about what program created it, what computer, etc.