Citation Management or Bibliographic Management is the practice of adopting strategies to organize secondary research sources (books, articles, websites, etc.). Therefore, citation management software is just a computer tool that facilitates the storage, organization, and use of secondary research sources.
Citation Managers like Zotero, Mendeley, and Endnote all utilize library metadata. Metadata is simply information about information. In this context, our information is the books, articles, websites, and other research sources in your collection. Metadata refers to things like author names, journal title, date published, subject, and source type.
A citation manager builds a personal database of all research sources you add to it. Because it uses metadata, the database is very well organized, and can be used to create citations while writing. Many citation managers have additional features that include sharing and additional tagging and organization tools.