Read only: Used by integrations and plugins. For example: PIM system ID to show the pairing between systems has succeeded. Values can be set programatically but not by the user through the interface.
Hidden: Same as above, mostly used by integrations and plugins to store data for individual assets, that the user shouldn't be bothered by. Hidden is used when the data is of no use to the editors, and will also usually be "read only" as well.
Link: Values in this property can be clicked and QBank will perform a search for media that have the same value in that property.
Ignore in suggestion: Mostly relevant for keyword/dropdown fields. Keywords & dropdown values are suggested when doing a search for example. Sometimes data is stored in these properties that aren't relevant for search, for example product numbers. Ignoring these types of non-relevant data makes the search input easier to use.