This is not related to game play as far as I know. Creatures such as Shivan Dragon, Victory's Herald and Glyph Keeper, and even enchantments like the double-sided Search for Azcanta, there is this shiny part near the bottom of the card. The only link that I have is that it is linked to rarity, because only cards that are rare and above have them, but not all of the cards that are rare and above do have them, and it is not related to premiums. Can anybody help me know what it is?
These are holograms, to prevent forgeries. They were introduced as part of a new card design in Magic Origins (2015):
Magic Origins (2015)
The second (and current) overall frame change was unveiled in Magic Origins. It was a more subtle change, adding some more (non-game related) information on the bottom of the card, which received a black colored background. Also, rare and mythic rare cards received a hologram sticker to prevent forgeries.