What does this oval symbol at the bottom of Magic cards mean?
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.

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Compare Shivan Dragon from M14 with the one from M15: