Metadata is data that describes your data. Metadata is used to structure actual data sets -- like the column headings of simple tabular data -- as well as to describe features of data sets. Some examples of metadata include information that answers the questions of when, who, what, why, how:
Sometimes metadata is contained in the data files produced by the software used to collect or analyze the data, other times it is included in a codebook or lab notebook. Every effort needs to be made to keep this information with the data set with which it is affiliated.
It provides the means for organizing and describing your data. Metadata facilitates data collection, processing, archiving, discovery, re-use and analysis.
Metadata standards not only facilitate use of your data in its native environment, but maximize its usability in other environments. For example, standardized metadata will allow you to more easily move your data from one data repository to another. Check into whether there are standards commonly employed by your department or your organization. Perhaps your research domain commonly employs a metadata standard. It may be that the repository into which you will be depositing your data has metadata requirements. You will have to do a little research.