In short, metadata is data about data:
There are three types of metadata described by the National Information Standards Organization (NISO) in their guide Understanding Metadata:
Descriptive metadata elements are used to describe the content and context of your data, such as title, author, and date created.
Structural metadata elements convey physical structure of the data. For datasets, examples would include table of contents, pages of data, the codebook, tables, and their relation to one another.
Adminstrative metadata elements provide managerial information about your data. These can include elements such as how the data was created (e.g. survey, SPSS analysis) who can access it, and any intellectual property information.
These are some basic metadata elements to use to document your data regardless of discipline. Please refer to some of the links under Resources for a detailed description of the elements.