What is Metadata?
Metadata is "data about data." This is a circular definition; so to simplify, think of a book you like. The title of that book is a piece of metadata; also the author, date published, etc.
Pieces of information about an item are called metadata fields (think of the title of a row in a spreadsheet), i.e. "Title," "Author," "Date," etc.
A group or list of those fields that apply to one item (think of a row in a spreadsheet) equals a metadata record.
Often, these records are formatted in a specific way, based on a metadata standard. A standard is a recognized and established format for the recording of metadata records.
For example, Dublin Core, a simple and effective standard, requires that each record include, at minimum these 15 elements:
(These elements are explained here.)
All this is done in order to contextualize the information stored electronically so that other people, as well as computers, can find and understand your data in the future. This is because indexes, catalogs and search engines run searches on metadata, not data. If you want others to find your data, you first have to create metadata.