they are objects with a rainbow sheen that unlock a box that probably contains a portal to the human world which can be accessed at any time and is solely for marketing purposes
it more likely contains an element-like object, or some mystical aura, that will transfer to the ponies and give them rainbow power. then celestia rewards twilight with a rainbow kingdom.
something about this will be related to the movie, but not directly. similar to the first, where twilight was worrying about being a leader then her element of magic crown was stolen.
if pony-twilight is even involved in this one, it'll likely be another situation like that. they're playing a battle of the bands concert-type of thing, so i'm not sure what in hell it has to do with any of that. regardless, just like the first movie, it'll be another standalone "episode" where the finale isn't related nor is the following season's premiere