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"Metadata is a love note to the future" (Jason Scott, @textfiles)
...to your future self, and to others who still want to make sense of your data in a few years.
Metadata can be considered as data about data and summarize all necessary information about data in a structured way to make the data understandable and machine readable. They should at least contain information on the data collector/owner, the content of the data, time and place of data collection, and the collection method - WHO? WHAT? WHEN? WHERE? HOW?.
For further information see here.
See also:
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