"Data" are the recorded, factual material commonly accepted in the scientific community as necessary to validate research findings.
"Metadata" is essentially description or documentation about data - e.g., "title," "format," "specimen," "date," "location," etc.
Data that are well documented are more easily discovered and re-discovered by others interested in your data. Complying with an accepted standard will also help in the indexing and retrieval of data.
An important best practice for creating metadata is to use a controlled vocabulary (definition linked below), a standardized terminology for your community of interest.