Metadata, in its different forms, should accompany your data when you publish it. Metadata conveys the information needed to find, understand, and use your data.
There are three general categories of this information:
Administrative: licensing information; who can use the data and under what conditions.
Technical: the technical considerations and requirements for working with the data.
Descriptive: the who, what, when, where, why and how of your data; descriptions of the contents of the data/data collection; the subject of the data.
Data repositories may support different forms of metadata, from structured information that is entered via a form and is mapped to a metadata schema, to depositing readme.txt files, data dictionaries and code books.
Most data repositories provide information on the standards they use. Some have detailed instructions, and some may even provide data curation support, to augment and refine the information needed by humans (and computers) to find, understand, link to, and use your data.