When adding a dropdown list attribute to the system you can optionally mark it as dependent on another 'parent' dropdown attribute. Dropdowns that are dependent on a parent dropdowns are referred to as child dropdowns.
These child dropdowns can have values added to them that have one or more parent list values in the parent dropdown.
So as an example...
You could add a new dropdown list attribute called 'Actors'. Then add another called 'Movies' that is dependant on the 'Actors' list as it's parent.
Then as you add movies to the second dropdown you can pick one or more actors that are parents of the movie.
This setup would mean that when a user uploads an asset and selects a certain value from the Actors dropdown list then the list of available options in the Movies attribute would be restricted to movies that featured the selected Actor.
i.e. selecting Tom Cruise from the Actors list would automatically filter the Movies list to only show Top Gun, Cocktail, Mission Impossible etc.