Metadata is the information that is exchanged between Pantavisor and the outside world, either remotely or locally, that is not attached to a revision itself.
Metadata is just a list of key-value pairs of string type.
This type of metadata is the one sent by Pantavisor or any of the running containers to expose internal information. Check the list of key-value pairs that Pantavisor sends.
This type of metadata is the one sent by the user so Pantavisor or any of the containers can consume it. Check out the list of user metadata consumed by Pantavisor.