Metadata is a general term for data that was not part of the original database structure. This can be anything you might want to use, but also requires you to provide the coding that detects and uses that data, because the maker's engine will not use it itself.
Easiest example: actor gender
By default the actors have no gender, but you can use metadata to define whether they're male or female, and then use a script line in a conditional branch to have events react differently to that data.